ARM: dts: fix PMC compatible

Make each SoC dtsi use its soc specific PMc compatible string. This solves
a potential issue on at91sam9261 and at91sam9263 when using suspend to RAM

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>