ARM: at91/pm: rename function name: at91_slow_clock() --> at91_pm_suspend_sram_fn()

As the file name is renamed, rename the function name at91_slow_clock()
--> at91_pm_suspend_sram_fn(), rename the function handler's name at the
same time.

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>
[nicolas.ferre@atmel.com: little update of the commit message]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>