LoongArch: Fix built-in DTB detection

fdt_check_header(__dtb_start) will always success because kernel
provides a dummy dtb, and by coincidence __dtb_start clashed with
entry of this dummy dtb. The consequence is fdt passed from firmware
will never be taken.

Fix by trying to utilise __dtb_start only when CONFIG_BUILTIN_DTB is
enabled.

Cc: stable@vger.kernel.org
Fixes: 7b937cc2

 ("of: Create of_root if no dtb provided by firmware")
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>