dt-bindings: clk: axg-clkc: add MIPI DSI Host clock binding

Add the clock ID for the MIPI DSI Host clock.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200915124553.8056-3-narmstrong@baylibre.com