ARM: at91/pm_slowclock: create the procedure to handle the sdram self-refresh

To decrease the duplicated code, create the procedure
to contain both activing and exiting the sdram self-refresh mode.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Tested-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>