arm: Use asm-generic/io.h

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

This has only been build-tested, feedback from architecture maintainers
is welcome.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Reviewed-by: Simon Glass <sjg@chromium.org>