davinci: DA850/OMAP-L138: add voltage regulation support

This patch adds support for regulating the CVDD voltage for the
DA850/OMAP-L138 platform.

The CVDD min and max values for each OPP have been obtained from
section 5.2 "Recommended Operating Conditions" of SPRS586

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>