gpiolib: Disallow identical line names in the same chip

We need to make this namespace hierarchical: at least do not
allow two lines on the same chip to have the same name, this
is just too much flexibility. If we name a line on a chip,
name it uniquely on that chip.

This does not affect device tree and other gpiochips that
get named from device properties: the uniqueness
per-chip however affect all hotplugged devices such as
GPIO expanders on USB.

Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20201212003447.238474-1-linus.walleij@linaro.org
[Dropped warning for globally unique]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
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ChangeLog v1->v2:
- Do NOT enforce unique line names on device tree instances.
  Devicetrees know what they are doing and will make sure
  not to create conflicting naming.