mfd: as3722 Update VSEL access control

mfd: as3722 Update VSEL access control

- Added an option to mark VSEL registers as volatile by default.
This option should be set either in board platform data, or in device
tree node of the regulator controlled by hardware mechanism underneath
s/w driver.

- Replaced volatile registers table with callback that checks access
attribute at run-time. Added another callback to switch VSEL registers
attributes between volatile and cached.

- Set persistent property for VSEL of all SD regulators (persistent
value is zero). This would allow to use direct write for setting the
voltage selector instead of read-modify-write.

Bug 1454969

Change-Id: I9a434fa8568dc2717a81ae6ecf3891abb903befa
Signed-off-by: Alex Frid <afrid@nvidia.com>
Reviewed-on: http://git-master/r/367357
Reviewed-by: Laxman Dewangan <ldewangan@nvidia.com>