ARM: dts: bcm283x: Enable HDMI at board level

There might be headless setups of the Compute Module without HDMI,
so better enable HDMI at board level. Btw this allows moving HDMI
base definition into upcoming bcm2835-common.dtsi.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Eric Anholt <eric@anholt.net>