drm: sti: add Mixer

Mixer hardware IP is responsible of mixing the different inputs layers.
Z-order is managed by the mixer.
We could 2 mixers: one for main path and one for auxillary path

Mixers are part of Compositor hardware block

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>