Add directives to get and set the priority of an interrupt vector.
Implement the directives for the following BSP families:
* arm/lpc24xx
* arm/lpc32xx
* powerpc/mpc55xxevb
* powerpc/qoriq
Implement the directives for the following interrupt controllers:
* GICv2 and GICv3 (arm and aarch64)
* NVIC (arm)
* PLIC (riscv)
Update #5002.
ARM's GICv2 is configurable and its attributes vary between
implementations including omission of specific interrupts. This allows
BSPs to accomodate those varying implementations with customized
attribute sets.