mfd: arizona: Factor out SYSCLK enable from wm5102 hardware patch

wm5102 applies a custom hardware boot sequence, for this the SYSCLK
needs to be enabled. This patch factors out the code that enables
SYSCLK for this sequence such that it can be used for other boot time
operations that require SYSCLK.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>