clk: tegra: add clock support for Tegra20

Add Tegra20 clock support based on common clock framework.

Signed-off-by: Prashant Gaikwad <pgaikwad@nvidia.com>
[swarren: s/1GHz/100MHz/ in call to tegra_clk_plle() to fix PCIe,
implemented KBC clock, ensure all OF lookups return valid cookies i.e.
an explicit error pointer or valid pointer not NULL, adapt to renames
in earlier patches, fixed some checkpatch issues.]
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>