sh: Provide unxlate_dev_mem_ptr() in asm/io.h

The unxlate_dev_mem_ptr() function has no prototype on the sh architecture
which does not include asm-generic/io.h. This results in the following
build failure:

drivers/char/mem.c: In function 'read_mem':
drivers/char/mem.c:164:25: error: implicit declaration of function 'unxlate_dev_mem_ptr'

This compile error is now seen because commit 99b619b3 ("mips: provide
unxlate_dev_mem_ptr() in asm/io.h") removed the weak function which was
previously in place to handle this problem.

Add a trivial macro to the sh header to provide the now missing dummy
function.

Fixes: 99b619b3

 ("mips: provide unxlate_dev_mem_ptr() in asm/io.h")
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/20230704190144.2888679-1-linux@roeck-us.net
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>