clk: tegra: Mark Memory Controller clock as critical

Memory Controller should be always-on. Currently the sibling EMC clock is
marked as critical, let's mark MC clock too for consistency.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-By: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>