regulator: pm8921-regulator: Add regulator driver for PM8921

Create a regulator driver to control all regulators on the Qualcomm
PM8921 PMIC chip.  This chip contains many different types of
regulators with a wide range of abilities and voltage ranges.

Eight different regulator types are available on the PM8921.  These
are managed via 7 different type values in the driver:

LDO      - low drop out regulator (supports both NMOS and PMOS LDOs)
NLDO1200 - 1.2A NMOS LDO (different control structure than other LDOs)
SMPS     - switched-mode power supply
FTSMPS   - fast transient SMPS
VS       - voltage switch
VS300    - 300mA voltage switch (different control structure than
             other switches)
NCP      - negative charge pump

The driver interfaces with the PMIC using Qualcomm's SSBI bus.
Calls to this bus are abtracted through the pm8xxx_readb/writeb API.

Change-Id: I01fb755c6be8e3f32c86fef079b5740edfc39f14
Signed-off-by: David Collins <collinsd@codeaurora.org>
[sboyd: Take only pm8921-core bits]