Introduce SPR_KAIVB for interrupt vector table relocation.
The PowerPC ISA allows for some amount of interrupt vector table relocation, but it is defined in the context of a hypervisor.
Kestrel/microwatt does not implement hypervisor, and so lacks the associated registers and logic.
Introduce a simple register to set the interrupt vector table base. Kestrel arch interrupt vector base: KAIVB.
Provides mapping of 48 bits (bits 14:61) of a 16KiB aligned table.