ARMV7: OMAP: add convenience function to set TWL4030 regulator voltages

ARMV7: OMAP: add convenience function to set TWL4030 regulator voltages

This patch adds a function to allow one to easily set the target
voltage for the TWL4030 regulators.  It also modifies the existing
code to use this new function.  Applicable definitions are moved
out of the driver file and into the header file so that they are
generally accessible

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>