ARM: at91/pm: flush data cache and clean, invalidate and disable the L2 cache

Flush data cache, and clean, invalidate and disable the L2 cache before going to suspend.
Restore the L2 cache configuration and re-enable the L2 cache after waking up.

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>