openrisc: io: Fixup defines and move include to the end

This didn't seem to cause any issues, but while working on fixing up
sparse annotations for OpenRISC I noticed this.  This patch moves the
include of asm-generic/io.h to the end of the file.  Also, we add
defines of ioremap and iounmap, that way we don't get duplicate
definitions from asm-generic/io.h.

Signed-off-by: Stafford Horne <shorne@gmail.com>