MIPS: generic: Use __initconst for const init data

Fix the following checkpatch errors:

$ ./scripts/checkpatch.pl --no-tree -f arch/mips/generic/init.c
ERROR: Use of const init definition must use __initconst
#23: FILE: arch/mips/generic/init.c:23:
+static __initdata const void *fdt;

ERROR: Use of const init definition must use __initconst
#24: FILE: arch/mips/generic/init.c:24:
+static __initdata const struct mips_machine *mach;

ERROR: Use of const init definition must use __initconst
#25: FILE: arch/mips/generic/init.c:25:
+static __initdata const void *mach_match_data;

Fixes: eed0eabd ("MIPS: generic: Introduce generic DT-based board support")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org