memory: tegra: Refashion EMC debugfs interface on Tegra124

The current debugfs interface is only partially useful. While it allows
listing supported frequencies and testing individual clock rates, it is
limited in that it can't be used to restrict the range of frequencies
that the driver is allowed to set. This is something we may want to use
to test adaptive scaling once that's implemented.

Signed-off-by: Thierry Reding <treding@nvidia.com>