dt-bindings: mfd: ti,keystone-devctrl: Convert to DT schema

Convert the Texas Instruments Keystone Device State Control binding from
legacy text format to DT schema.

During the conversion, the following updates were made:
- Add "simple-mfd" to compatibles matching existing dts usage.
- Add required bus properties #address-cells, #size-cells, and ranges
  which were missing in text binding.
- Document child nodes gpio, interrupt-controller, and reset-controller.

Signed-off-by: Bhargav Joshi <j.bhargav.u@gmail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260901-ti-keystone-devctrl-v4-2-2922f2a44792@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>