sandbox: Use asm-generic/io.h

Convert the sandbox architecture to make use of the new asm-generic/io.h
to provide address mapping functions. As sandbox actually performs
non-identity mapping between physical & virtual addresses we can't
simply make use of the generic mapping functions, but are able to
implement phys_to_virt() & make use of it from map_physmem().

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>