mfd: pm8xxx-mpp: Add pm8xxx MPP driver

Add support for multi-purpose pins (MPPs) on Qualcomm PM8xxx
PMIC chips.

PM8xxx MPPs can be configured as digital or analog inputs or
outputs, current sinks, or buffers.

Note that mpp pins appear as gpio lines to the kernel. However they
are implemented separately from the pmic's gpio driver as
mpps have different configuration attributes and have different
register controls than the pmic's gpio controller. Basically they are
different set of pins.

Change-Id: Iab39b2f7c2ba3f35ef6ac74d37ee7add8c70681f
Signed-off-by: David Collins <collinsd@codeaurora.org>