mfd: rtsx: Add output voltage switch hook

Different card reader has different method to switch output voltage,
add this callback to let the card reader implement its individual switch
function.
This is needed as rtl8411 has a specific switch output voltage procedure.

Signed-off-by: Wei WANG <wei_wang@realsil.com.cn>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>