arm: Add a 'clear PPI' method to gic_pl390

arm: Add a 'clear PPI' method to gic_pl390

The underlying assumption that all PPIs must be edge-triggered is
strained when the architected timers and VGIC interfaces make
level-behaviour observable. For example, a virtual timer interrupt
'goes away' when the hypervisor is entered and the vtimer is disabled;
this requires a PPI to be de-activated.

The new method simply clears the interrupt pending state.