drm: sti: add HDMI driver

Add driver for HDMI output.
HDMI PHY registers are mixed into HDMI device registers
and their is only one IRQ for all this hardware block.
That is why PHYs aren't using phy framework but only a
thin hdmi_phy_ops structure with start and stop functions.

HDMI driver is mapped on drm_bridge and drm_connector structures.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>