ARM: tegra: Add missing HDMI +5V regulator

Commit fb50a116 ("drm/tegra: hdmi - Add connector supply support")
introduced a new supply for HDMI connectors that is used to control the
voltage on the +5V pin. Not all boards have had the corresponding supply
added to their device tree files, causing the following warning message
during boot:

	[    0.859698] 54280000.hdmi supply hdmi not found, using dummy regulator

Add such a regulator to the Seaboard DTS to enable the driver to control
this voltage and get rid of the warning.

Reported-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>