OMAP3: remove OPP interfaces from OMAP PM layer

With new OPP layer, OPP users will access OPP API directly instead of
using OMAP PM layer, so remove all notions of OPPs from the OMAP PM
layer.

Acked-by: Paul Walmsley <paul@pwsan.com>

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>