memory: tegra20-emc: Add devfreq support

Add devfreq support to the Tegra20 EMC driver. Memory utilization
statistics will be periodically polled from the memory controller and
appropriate minimum clock rate will be selected by the devfreq governor.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://lore.kernel.org/r/20201111011456.7875-5-digetx@gmail.com


Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>