ARM: Tegra: Add smp_twd clock for Tegra20

Clockevent's frequency is changed upon cpufreq change
notification. It fetches local timer's rate to update the
clockevent frequency. This patch adds local timer clock
for Tegra20.

Signed-off-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>