mfd: arizona: Factor out DCVDD isolation control

Currently DCVDD isolation is enabled and disabled for
runtime_suspend and runtime_resume. Future codecs will not
have the isolation control so to prepare for these codecs
this patch factors out the isolation control allowing it to
be called as needed in the existing codec-specific switch cases.

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