ARM: at91/pm: remove CONFIG_AT91_SLOW_CLOCK config option

The slow clock always exists, selecting CONFIG_AT91_SLOW_CLOCK config
is unnecessary for the suspend to memory mode.
For this mode the master clock should always switch to the slow 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>