memory: tegra: Introduce Tegra30 EMC driver

Introduce driver for the External Memory Controller (EMC) found on Tegra30
chips, it controls the external DRAM on the board. The purpose of this
driver is to program memory timing for external memory on the EMC clock
rate change.

Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Tested-by: Peter Geis <pgwipeout@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>