omap: opp: add OMAP3 OPP table data and common init

Add OPP data for OMAP34xx and OMAP36xx and initialization functions
to populate OPP tables based on current SoC.
introduce an OMAP generic opp initialization routine which OMAP3
and OMAP4+ SoCs can use to register their OPP definitions.

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>