parisc: Add PDC locking functions for rendezvous code

Add pdc_cpu_rendezvous_lock() and pdc_cpu_rendezvous_unlock()
to lock PDC while CPU is transitioning into rendezvous state.
This is needed, because the transition phase may take up to 8 seconds.

Add pdc_pat_get_PDC_entrypoint() to get PDC entry point for current CPU.

Signed-off-by: Helge Deller <deller@gmx.de>