irqchip/hip04: Configure IPIs as standard interrupts

In order to switch the hip04 driver to provide standard interrupts
for IPIs, rework the way interrupts are allocated, making sure
the irqdomain covers the SGIs as well as the rest of the interrupt
range.

The driver is otherwise so old-school that it creates all interrupts
upfront (duh!), so there is hardly anything else to change, apart
from communicating the IPIs to the arch code.

Signed-off-by: Marc Zyngier <maz@kernel.org>