xtensa: move parse_tag_fdt out of #ifdef CONFIG_BLK_DEV_INITRD

FDT tag parsing is not related to whether BLK_DEV_INITRD is configured
or not, move it out of the corresponding #ifdef/#endif block.
This fixes passing external FDT to the kernel configured w/o
BLK_DEV_INITRD support.

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>