mfd: TWL6030: add ADC driver

This driver takes the TWL4030 ADC driver as a template and implements
the ability to fetch a result from any channel on the mux.  It does
not implement the other features in the TWL4030 ADC driver.

Change-Id: If9150b2386e2c11754f40b1de7b28f1680a5730e
Signed-off-by: Marty Fouts <mfouts@sta.samsung.com>

Conflicts:
	drivers/mfd/Kconfig
	drivers/mfd/Makefile