arm64: dts: meson: kvim3: move hdmi to tdm a

The tdmout b is physically routed to the 40 pin header and the tdmout c
is routed to the m2 connector. It makes these interfaces poor
candidates to handle the HDMI 8ch i2s link (2ch i2s * 4 lanes) as it would
force the same link format on the related connectors.

Instead use the TDM A interface. This one is not routed to the outside
world on the vim3, so it can only be used for HDMI.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200421141814.639480-1-jbrunet@baylibre.com