PM / devfreq: tegra20: Relax Kconfig dependency

The Tegra EMC driver now could be compiled as a loadable kernel module.
Currently devfreq driver depends on the EMC/MC drivers in Kconfig, and
thus, devfreq is forced to be a kernel module if EMC is compiled as a
module. This build dependency could be relaxed since devfreq driver
checks MC/EMC presence on probe, allowing kernel configuration where
devfreq is a built-in driver and EMC driver is a loadable module.
This change puts Tegra20 devfreq Kconfig entry on a par with the Tegra30
devfreq entry.

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>