irqchip/gic-v3-its: Move ITS' ->pend_page allocation into an early CPU up hook

[ Upstream commit e083f14dc2e98ced872bf077b4d1cccf95b7e4f8 ]

The AP-GIC-starting hook allocates memory for the ->pend_page while the
CPU is started during boot-up. This callback is invoked on the target
CPU with disabled interrupts.
This does not work on -RT beacuse memory allocations are not possible
with disabled interrupts.
Move the memory allocation to an earlier hotplug step which invoked with
enabled interrupts on the boot CPU.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>