xtensa: Use asm-generic/io.h

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

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

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>