drm/omap: add dmm_read() and dmm_write() wrappers

This patch adds wrapper functions for readl() and writel(), dmm_read()
and dmm_write(), so that we can implement workaround for DRA7 errata
i878.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>