ARM: at91: pm: prepare for multiplatform

Split at91_pm_init() in three variants that are called by the respective SoCs
.init_machine. This allows to remove the of_machine_is_compatible() calls and
move at91_pm_init() out of arch_initcall() which is required for multiplatform.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>