-V3 Add MFD driver for TPS6507x family of multi-function chips and move TPS6507x regulator driver from being stand-alone to using the MFD driver.

Add MFD driver for TPS6507x family of multi-function chips.  Move TPS6507x
regulator driver from being stand-alone driver to using the MFD TPS6507x driver.
Signed-off-by: Todd Fischer <todd.fischer@ridgerun.com>