clk: tegra: Implement memory-controller clock

The memory controller clock runs either at half or the same frequency as
the EMC clock.

Reviewed-By: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>