arm: nvic: Use uint8_t for priority level

Since the value is treated as uint8_t internally anyway, just use
uint8_t for the input and output parameter types.

Jira: ZEP-1497

Change-Id: I61d68eb39cba5d82dad6ab7593b267c26e003d2b
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
(cherry picked from commit ae0ce206990c8ecb1f1084e1439392a46b07d766)