arm: better handling of IRQ priorities reserved by the kernel

There are now three flags that decide how many priorities are reserved
by the kernel, each one requiring one priority level: Zero Latency
Interrupts, BASEPRI locking (for SVC usage) and faults that are not at
priority -1, so that taking them in an ISR actually triggers the fault
synchronously.

Change-Id: I7f4d760c9110051aeb82dcfd8cd68026a9b74b54
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>