xtensa: move built-in PIC to drivers/irqchip

Extract xtensa built-in interrupt controller implementation from
xtensa/kernel/irq.c and move it to other irqchips, providing way to
instantiate it from the device tree.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>