Add a macro that gets the physical address of a memory mapped device
register from a bus space resource.

Note that this macro is just for ARM, and is intended to have a short
lifespan.  The DMA engines in some SoCs need the physical address of a
memory-mapped device register as one of the arguments for the transfer.
Several scattered ad-hoc solutions have been converted to use this macro,
which now also serves to mark the places where a more complete fix needs
to be applied (after that fix has been designed).