power: reset: at91-poweroff: lookup for proper pmc dt node for sam9x7

Use sam9x7 pmc's compatible to lookup for in the SHDWC driver.

Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Link: https://lore.kernel.org/r/20241010120404.92893-1-varshini.rajendran@microchip.com


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>