irq: cortex-r: Fix wrong irq enabling
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 <ca4506c8c654a4ccf75802e86b5d162c1e96f065@baylibre.com>

zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr