dt-bindings: mfd: atmel: Convert to YAML schema

dt-bindings: mfd: atmel: Convert to YAML schema

Changes during conversion:

The text binding is misleading, add a fallback `atmel,at91sam9260-gpbr`
for both `microchip,sam9x60-gpbr` and `microchip,sam9x7-gpbr` which is
missing in old binding and `microchip,sam9x60-gpbr` is not a fallback
for `microchip,sam9x7-gpbr`.

Signed-off-by: Charan Pedumuru <charan.pedumuru@microchip.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20241219-gpbr-v1-1-e19a562ebf81@microchip.com
Signed-off-by: Lee Jones <lee@kernel.org>