ARM: at91: fix PM initialization for newer SoCs

Newer SoCs: at91sam9x5, at91sam9n12, sama5d3 and sama5d4 embed a DDR controller
and have a different PMC status register layout than the at91sam9g45. Create
another at91_sam9x5_pm_init() function to match this compatibility.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>