The Amlogic SM1 DynamIQ Shared Unit has a dedicated clock tree similar to
the CPU clock tree with a supplementaty mux to select the CPU0 clock
instead.

Leave this as read-only since it's set up by the early boot stages.
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>