rockchip: video: rk3399: add HDMI TX support on the RK3399

This commit enables the RK3399 HDMI TX, which is very similar to the
one found on the RK3288.  As requested by Simon, this splits the HDMI
driver into a SOC-specific portion (rk3399_hdmi.c, rk3288_hdmi.c) and
a common portion (rk_hdmi.c).

Note that the I2C communication for reading the EDID works well with
the default settings, but does not with the alternate settings used on
the RK3288... this configuration aspect is reflected by the driverdata
for the RK3399 driver.

Having some sort of DTS-based configuration for the regulator
dependencies would be nice for the future, but for now we simply use
lists of regulator names (also via driverdata) that we probe.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>