microblaze: Use asm-generic/io.h

Convert the microblaze architecture to make use of the new
asm-generic/io.h to provide address mapping functions. As the generic
implementations are suitable for microblaze this is primarily a matter
of removing code.

Feedback from architecture maintainers is welcome.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Michal Simek <monstr@monstr.eu>