dt-bindings: mfd: atmel-matrix: Add microchip,sam9x7-matrix

Add microchip,sam9x7-matrix to DT bindings documentation.

Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230728102539.266085-1-varshini.rajendran@microchip.com


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