arc: Use asm-generic/io.h

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

Feedback from architecture maintainers is welcome.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Alexey Brodkin <alexey.brodkin@synopsys.com>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>