mfd: Add 88PM8607 driver

This adds a core driver for 88PM8607 found in Marvell DKB development
platform. This driver is a proxy for all accesses to 88PM8607
sub-drivers which will be merged on top of this one, RTC, regulators,
battery and so on.

This chip is manufactured by Marvell.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>