dt-bindings: atmel-nand: add microchip,sam9x7-pmecc

Add microchip,sam9x7-pmecc to DT bindings documentation.

Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240223172520.671940-1-varshini.rajendran@microchip.com