soc/tegra: pmc: Parameterize driver

Parameterize some aspects of the driver in preparation for Tegra186 PMC
support. Initially the Tegra186 driver had been split off into an extra
driver, but it turns out the backwards-compatibility break isn't as bad
as originally assumed, so with a little parameterization the same code
can be used to keep supporting all SoC generations.

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