mfd: add AB4500 driver

This adds core driver support for AB4500 mixed signal
multimedia & power management chip. This connects to U8500
on the SSP (pl022) and exports read/write functions for
the device to get access to this chip. This also registers
the client devices and sets the parent.

Signed-off-by: srinidhi kasagar <srinidhi.kasagar@stericsson.com>
Acked-by: Andrea Gallo <andrea.gallo@stericsson.com>
Reviewed-by: Mark Brown <broonie@sirena.org.uk>
Reviewed-by: Jean-Christophe <plagnioj@jcrosoft.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>