gpio: Leverage revocable for accessing struct gpio_chip

The underlying chip can be removed asynchronously.  `gdev->srcu` is used
to ensure the synchronization before accessing `gdev->chip`.

Revocable encapsulates the details.  Leverage revocable for accessing
the struct gpio_chip and remove the `gdev->srcu`.

Tested-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
---
Test:

Prepare gpiod-test.
(https://libgpiod.readthedocs.io/en/master/building.html#building)
$ cd libgpiod-2.3.1
$ mkdir build && cd build
$ meson setup .. -Dtools=enabled
$ ninja

Prepare kernel.
$ cat >test_config <<EOF
CONFIG_IKCONFIG=y
CONFIG_IKCONFIG_PROC=y
CONFIG_KASAN=y
CONFIG_GPIOLIB=y
CONFIG_GPIO_SYSFS=y
CONFIG_GPIO_SIM=y
CONFIG_EXPERT=y
CONFIG_PROVE_LOCKING=y
CONFIG_PANIC_ON_OOPS=y
CONFIG_DEBUG_KERNEL=y
CONFIG_LOCKDEP=y
EOF
$ export KBUILD_OUTPUT=build
$ vng -k -f test_config && vng -b

Run gpiod-test.
$ vng
$ ./libgpiod-2.3.1/build/tests/gpiod-test
$ time ./libgpiod-2.3.1/build/tests/gpiod-test >/dev/null 2>&1
---
v14:
- Rebase onto next-20261007.
  - Fix conflicts with 1feb5d39b05a ("gpio: cdev: fix kernel stack leak
    to user-space in error path")

v13: https://lore.kernel.org/all/20260912123529.7951-4-tzungbi@kernel.org
- Call revocable_deinit() instead of revocable_put() as it uses embedded
  allocation.

v12: https://lore.kernel.org/all/20260908134223.13638-4-tzungbi@kernel.org
- Rebase onto next-20260907.
- Fix conflicts mainly with aa7e8b7ef031 ("gpio: core: fix
  const-correctness of gpio_chip_guard").
- Fix error unwinding in gpiochip_add_data_with_key() by initializing
  chip_rev before device_initialize() and ensuring both references are
  dropped on error.

v11: https://lore.kernel.org/all/20260513091043.6766-4-tzungbi@kernel.org
- Add Tested-by tag.
- Squash
  - https://lore.kernel.org/all/20260508105448.31799-5-tzungbi@kernel.org
  - https://lore.kernel.org/all/20260508105448.31799-6-tzungbi@kernel.org
  - https://lore.kernel.org/all/20260508105448.31799-7-tzungbi@kernel.org
  - https://lore.kernel.org/all/20260508105448.31799-8-tzungbi@kernel.org

v10: https://lore.kernel.org/all/20260508105448.31799-4-tzungbi@kernel.org
- Change revocable API usages accordingly.

v9: https://lore.kernel.org/all/20260427135841.96266-4-tzungbi@kernel.org
- New to the series.
- Use static allocated resource provider.
- Rename "chip_rp" -> "chip_rev".

v4 - v8:
- Doesn't exist.

v3: https://lore.kernel.org/all/20260213092958.864411-8-tzungbi@kernel.org
- Change revocable API usages accordingly.

v2: https://lore.kernel.org/all/20260203061059.975605-8-tzungbi@kernel.org
- Change usages accordingly after applying
  https://lore.kernel.org/all/20260129143733.45618-2-tzungbi@kernel.org.
  - Add __rcu for `chip_rp`.
  - Pass pointer of pointer to revocable_provider_revoke().
- Rebase accordingly after applying
  https://lore.kernel.org/all/20260203060210.972243-1-tzungbi@kernel.org.

v1: https://lore.kernel.org/all/20260116081036.352286-13-tzungbi@kernel.org