dt-bindings: serial: atmel,at91-usart: add compatible for sam9x7.

Add sam9x7 compatible to DT bindings documentation.

Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240729070636.1990368-1-varshini.rajendran@microchip.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>