ARM: OMAP4+: Add support for dynamically selecting OPPs

It can be expected that different paper spins of a SoC can have
different definitions for OPP and can have their own constraints
on the boot up OPP for each voltage rail. In order to have this
flexibility, add support for dynamically selecting the OPP voltage
based on the board to handle any such exceptions.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>