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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.