Some Rockchip SoCs support HDMI output. Add a display driver for this so
that these displays can be used on supported boards.

Unfortunately this driver is not fully functional. It cannot reliably read
EDID information over HDMI. This seems to be due to the clocks being
incorrect - the I2C bus speed appears to be up to 100x slower than the
clock settings indicate. The root cause may be in the clock logic.

Signed-off-by: Simon Glass <sjg@chromium.org>