rockchip: video: split RK3288-specific part off from rk_hdmi

To prepare for the addition of RK3399 HDMI support, the HDMI driver is
refactored and broken into a chip-specific and a generic part.  This
change adds the internal interfaces, makes common/reusable functions
externally visible and splits the RK3288 driver into a separate file.

For the probing of regulators, we reuse the infrastructure created
during the VOP refactoring... i.e. we simply call into the helper
function defined for the VOP.

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