lib: utils/fdt: Require match data to be const

Match data stores hardware attributes which do not change at runtime, so
it does not need to be mutable. Make it const.

Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>