arm/irq_manage: change assert to allow sharing prio with PendSV

Zephyr uses the last priority level for the PendSV exception, but
sharing prio can still be useful on systems with a reduced set of
priorities, like Cortex-M0/M0+.

Change-Id: I767527419dcd8f67c2b406756b9208afd3b96de0
Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>