asm-generic/io.h: add big endian versions of io{read,write}{16,32}

The asm-generic/iomap.h provides these functions already, but the
non-generic fallback defines do not.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>