dt-bindings: gpio: convert Atmel GPIO to json-schema

Convert the Atmel GPIO controller binding document to DT schema format
using json-schema.
The at91 pinctrl driver uses "atmel,at91rm9200-gpio" compatible string
to find the number of active GPIO banks and identify the pinmux nodes.
"atmel,at91sam9x5-gpio" and "microchip,sam9x60-gpio" have additional
registers to handle drive-strength, slew-rate,  pull-down to drive the
pinmux configs.
The new compatible string "microchip,sam9x7-gpio" is added.

Signed-off-by: Manikandan Muralidharan <manikandan.m@microchip.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240709092354.191643-5-manikandan.m@microchip.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>