mfd: Add TPS6507x support

TPS6507x are multi function (PM, touchscreen) chipsets from TI.
This commit also changes the corresponding regulator driver from being
standalone to an MFD subdevice.

Signed-off-by: Todd Fischer <todd.fischer@ridgerun.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>