Revert "MIPS: Octeon: Remove special handling of CONFIG_MIPS_ELF_APPENDED_DTB=y"

This reverts commit d9df9fb9.

For the OCTEON boards, it need to patch the built-in DTB before using
it. Previously it judges if it is a built-in DTB by checking
fw_passed_dtb. But after commit 37e5c69f ("MIPS: head.S: Init
fw_passed_dtb to builtin DTB", the fw_passed_dtb is initialized even
when using built-in DTB. This causes the OCTEON boards boot broken due
to an unpatched built-in DTB is used. Revert the commit d9df9fb9 to
restore the codes before the fw_passed_dtb is used and then fix this
issue.

Fixed: 37e5c69f ("MIPS: head.S: Init fw_passed_dtb to builtin DTB")
Cc: stable@vger.kernel.org
Suggested-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>