staging: drm/imx: add support for ARCH_MULTIPLATFORM

No dependency on plat headers, so only needs Kconfig update to
build for ARCH_MULTIPLATFORM.

Signed-off-by: Rob Clark <rob@ti.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>