ARM: dts: AM33XX beagle black: add pinmux and hdmi node to enable display

Enable the hdmi output and the LCD Controller on BeagleBone
Black. Also configure the correct pinmux for output of
video data from the SoC to the HDMI encoder.

Signed-off-by: Darren Etheridge <detheridge@ti.com>
Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>