clocksource: tegra20: use the device_node pointer passed to init

We've already matched the node, so use the node pointer passed in. The rtc
init was intermingled with the timer init, so split this out to a separate
init function.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>

Cc: John Stultz <johnstul@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>