ARM: at91/pm: rename file name: pm_slowclock.S --> pm_suspend.S

Because the sram function is used for both suspend to memory and the suspend
to standby mode, renaming is more elegant.

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>