mfd: PM8821: Add support for PM8821 IRQ controller

Add support for the irq controller in Qualcomm PM8821 pmic. The
interrupt controller provides control for MPPs configured as
interrupts in addition to other subdevice interrupts.

The PM8821 IRQ controller is simpler than Secure IRQ controller
in other PMIC4 family of chips, i.e. PM8921. Also, it does not adhere
to SSBI register layout of Secure IRQ controller. This driver follows
the SSBI register layout of PM8821 IRQ controller and  supports only
PM8821 IRQ controller.

The interrupt controller also provides a way to read the real time
status of an interrupt. This real time status is the only way one
can get the input values of gpio and mpp lines.

CRs-Fixed: 366276
Change-Id: Id4b9cbf42f296c26d4f8780590389bb2265e46c0
Signed-off-by: Jay Chokshi <jchokshi@codeaurora.org>