irq: cortex-r: Fix wrong irq enabling

In the cortex-r port we are currently using GIC as a fake cascade
controller hooked to a fake parent IRQ #0. And in gic_init() we use
IRQ_CONNECT() to connect this dummy IRQ.

Unfortunately this value is shifted and offset when calling
irq_set_priority_next_level() that tries to set the IRQ priority on a
value of 0xffffffff.

This value is offset again in gic_irq_set_priority() that actually sets
the priority on the PPI #31.

Fix this avoiding to set any priority for IRQ #0.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>