power: reset: at91-poweroff: lookup for proper PMC DT node

Driver has been enabled also for SAM9X60. At the moment the patch which did
this has been sent to mainline the PMC for SAM9X60 wasn't integrated.
SAM9X60 has a new PMC compatible (see commit 01e2113d
("clk: at91: add sam9x60 pmc driver")). Do to this we have to
look for proper PMC compatible here, in SHDWC driver.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>