rtc: tegra: Use consistent variable names and types

Many of the variables have redundant prefixes or suffixes. Drop all of
them where not necessary for context. Also make sure to use data types
consistently. For instance, values read from 32-bit register accessors
should be stored in u32.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>