backend.
The VME2chip can use this to translate a VMEbus irq to a cpu irq.
The VMEchip (on mvme147) can't deal with the VMEbus irq and cpu irq
being different so we just panic in that case for now.
deal with dynamic address modifier generation based on the CPU's
function code pins.
Also implement VMEbus slave mode for mvme147. (Not yet 100% working.)
These changes add support for:
o The MI VMEbus framework on both MVME147 and MVME167.
o Enhancements to the existing MD bus_space(9) implementation.
o Most of the bus_dma(9) API.
