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 99b619b37ae1 ("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: 99b619b37ae1 ("mips: provide unxlate_dev_mem_ptr() in asm/io.h")
Cc: Arnd Bergmann <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Reviewed-by: John Paul Adrian Glaubitz <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: John Paul Adrian Glaubitz <[e-mail]>

arch/sh/include/asm/io.h

···
  * access
  */
 #define xlate_dev_mem_ptr(p)	__va(p)
+#define unxlate_dev_mem_ptr(p, v) do { } while (0)
 
 #define ARCH_HAS_VALID_PHYS_ADDR_RANGE
 int valid_phys_addr_range(phys_addr_t addr, size_t size);