GPO regulators are digital outputs that can be enabled or disabled by a
dedicated bit in mc13783 POWERMISC register.
In this family can be count in also Power Gates (PWGT1 and 2): enabled by
a dedicated pin a Power Gate is an hardware driven supply where the output
(PWGTnDRV) follow this law:

 Bit PWGTxSPIEN | Pin PWGTxEN | PWGTxDRV |  Read Back
   0 = default  |             |          | PWGTxSPIEN
 ---------------+-------------+----------+------------
       1        |      x      |   Low    |     0
       0        |      0      |   High   |     1
       0        |      1      |   Low    |     0

As read back value of control bit reflects the PWGTxDRV state (not the
control value previously written) and mc13783 POWERMISC register contain
only regulator related bits, a dedicated function to manage these bits is
created here with the aim of tracing the real value of PWGTxSPIEN bits
and reproduce it on next writes.

All POWERMISC users _must_ use the new function to not accidentally
disable Power Gates supplies.

v2 changes:
-Better utilization of abstraction layers.
-Voltage query support. GPO's and PWGTxDRV are fixed voltage regulator
 with voltage value of 3.1V and 5.5V respectively.
Signed-off-by: Alberto Panizzo <maramaopercheseimorto@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>