clk: meson: g12a: add MIPI DSI Host Pixel Clock

This adds the MIPI DSI Host Pixel Clock, unlike AXG, the pixel clock can be different
from the VPU ENCL output clock to feed the DSI Host controller with a different clock rate.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20201126141600.2084586-3-narmstrong@baylibre.com