PM / devfreq: tegra20: Deprecate in a favor of emc-stat based driver

Remove tegra20-devfreq in order to replace it with a EMC_STAT based
devfreq driver. Previously we were going to use MC_STAT based
tegra20-devfreq driver because EMC_STAT wasn't working properly, but
now that problem is resolved. This resolves complications imposed by
the removed driver since it was depending on both EMC and MC drivers
simultaneously.

Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>