ARM: at91/pm_suspend: add the WFI instruction support for ARMv7

Add the WFI instruction to make the cpu to the idle state.
In the meanwhile, disable the processor's clock.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Tested-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>