MIPS/head: Store ELF appended dtb in a global variable too

Since commit 15f37e15 ("MIPS: store the appended
dtb address in a variable"),
in kernels with MIPS_RAW_APPENDED_DTB=y, the early boot code detects
the dtb and stores it in the 'fw_passed_dtb' variable.

However, the dtb is not stored in 'fw_passed_dtb' in kernels with
MIPS_ELF_APPENDED_DTB=y.

Under MIPS_ELF_APPENDED_DTB=y, the dtb is also located in the
__appended_dtb section, so we just need to update the #ifdef.

This will allow to access the dtb in a more uniform way.

Fixes: 15f37e15 ("MIPS: store the appended dtb address in a variable")
Signed-off-by: Yasha Cherikovsky <yasha.che3@gmail.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20803/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org