m68k: Make sure {read,write}s[bwl]() are always defined

drivers/usb/musb/musb_io.h provides default implementations for
{read,write}s[bwl]() on most platforms, some of which will conflict soon
with platform-specific counterparts on m68k.

To avoid having to add more platform-specific checks to musb_io.h later,
make sure {read,write}s[bwl]() are always defined on m68k, and disable the
default implementations in musb_io.h on m68k, like is already done for
several other architectures.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Felipe Balbi <balbi@ti.com>