clk: meson: axg: add MIPI DSI Host clock

This adds the MIPI DSI Host clock, used to measure the signal timings
(ENC VSYNC or DW-MIPI-DSI eDPI timings).

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20200915124553.8056-5-narmstrong@baylibre.com