clk: tegra: divider: Mark Memory Controller clock as read-only

The Memory Controller (MC) clock rate can't be simply changed and nothing
in kernel need to change the rate, hence let's make the clock read-only.
This id also needed for the EMC driver because timing configuration may
require the MC clock diver to be disabled, that is handled by the EMC
clock / EMC driver integration and CLK framework shall not touch the
MC divider configuration on the EMC clock rate change.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>