OMAPDSS: HDMI: Add DRA7xx support

Add support for DRA7xx to the HDMI driver.

The HDMI block on DRA7xx is the same as on OMAP5, except we need to
enable and disable the HDMI PLL via the CONTROL module.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>