arm/tegra: prepare pinmux code for multiple tegra variants

This patch modifies the pinmux code to be useable for multiple tegra variants.
Some tegra20 specific constants will be replaced by variables which will be
initialized to the appropriate value at runtime.

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