ARM: tegra: clocks: Add emc scaling

Add clock ops on the emc peripheral clock that call into the
emc driver to update the memory controller registers for the
new frequency.  Tegra has an interlock between the clock
controller and the memory controller that prevents the new
register values from taking effect until the clock frequency
update occurs.

Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Colin Cross <ccross@android.com>