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: NThierry Reding <treding@nvidia.com>