ARM: tegra: Add external memory controller driver

The frequency memory bus on Tegra can be adjusted without
disabling accesses to memory by updating the memory
configuration registers from a per-board table, and then
changing the clock frequency.  The clock controller and
memory controller have an interlock that prevents the
new memory registers from taking effect until the
clock frequency change.

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