ARM: tegra: cleanup use of chipid register

The chipid register contains information about the Tegra variant and revision.
We want differentiate between Tegra variants for powergating and secondary
core bringup. This patch cleans up the reading and decoding of this register.
In subsequent patches the variant will exported as a global variable.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>