ARM: at91: pm: use SAM9X60 PMC's compatible

SAM9X60 PMC's has a different PMC. It was not integrated at the moment
commit 01c7031c ("ARM: at91: pm: initial PM support for SAM9X60")
was published.

Fixes: 01c7031c

 ("ARM: at91: pm: initial PM support for SAM9X60")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/1576062248-18514-2-git-send-email-claudiu.beznea@microchip.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>