drm: sti: enable auxiliary CRTC

For stih407 SoC enable the second mixer to get two CRTC.
Allow GPD planes and encoders to be connected to this new CRTC.
Cursor plane can only be set on first CRTC.
GPD clocks needed change the parent clock depending on which
CRTC GPD are used.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>