sh: mm: re-add lost __ref to ioremap_prot() to fix modpost warning

sh: mm: re-add lost __ref to ioremap_prot() to fix modpost warning

When __ioremap_caller() was replaced by ioremap_prot(), the __ref
annotation added in commit af1415314a4190b8 ("sh: Flag __ioremap_caller()
__init_refok.") was removed, causing a modpost warning:

    WARNING: modpost: vmlinux: section mismatch in reference: ioremap_prot+0x88 (section: .text) -> ioremap_fixed (section: .init.text)

ioremap_prot() calls ioremap_fixed() (which is marked __init), but only
before mem_init_done becomes true, so this is safe.  Hence fix this by
re-adding the lost __ref.

Link: https://lkml.kernel.org/r/20230911093850.1517389-1-geert+renesas@glider.be
Fixes: 0453c9a78015cb22 ("sh: mm: convert to GENERIC_IOREMAP")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Baoquan He <bhe@redhat.com>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Rich Felker <dalias@libc.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>