Make gicd_set_ipriorityr() actually write to the priority register.

Also correct callers of this function which want the highest priority
to use the value zero as this is the highest priority value according
to the ARM Generic Interrupt Controller Architecture Specification.

To make this easier to get right, we introduce defines for the lowest
and highest priorities for secure and non-secure interrupts.

Fixes ARM-software/tf-issues#21

Signed-off-by: Jon Medhurst <tixy@linaro.org>