sbc8641d: Fix PCI mapping concepts

Rename _BASE to _BUS, as it's actually a PCI bus address,
separate virtual and physical addresses into _VIRT and _PHYS,
and use each appopriately.  This makes the code easier to read
and understand, and facilitates mapping changes going forward.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>