mfd: pm8018: Add PMIC 8018 core driver

Add support for the Qualcomm PM8018 PMIC chip. The core driver
will communicate with the PMIC chip via the MSM SSBI bus.

Initial support is provided for: IRQ, GPIO, MPP, RTC, Power Key,
Misc, and Debug

Change-Id: I5787768603cb34bd3c9486d5c7d3fcf27a781ee9
Signed-off-by: David Collins <collinsd@codeaurora.org>