dt-bindings: clk: g12a-clkc: add DSI Pixel clock bindings

This adds the MIPI DSI Host Pixel Clock bindings.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20201126141600.2084586-2-narmstrong@baylibre.com