OMAP4: Register voltage PMIC parameters with the voltage layer

TWL6030 is the power IC used along with OMAP4 in OMAP4 SDPs,
blaze boards and panda boards. This patch registers the OMAP4
PMIC specific information with the voltage layer.
This also involves implementing a different formula for
voltage to vsel and vsel to voltage calculations from
TWL4030.

Signed-off-by: Thara Gopinath <thara@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>