dt-bindings: mfd: at91: Add SAM9X7 compatible string

Document sam9x7 DT for flexcom.

Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230728102515.265971-1-varshini.rajendran@microchip.com


Signed-off-by: Lee Jones <lee@kernel.org>