staging: drm/omap: add support for ARCH_MULTIPLATFORM

Remove usage of plat/cpu.h and get information from platform data
instead.  This enables omapdrm to be built with ARCH_MULTIPLATFORM.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>