ipq806x: fix AP148 boot from NOR

This patch allows AP148 to boot from NOR flash.

As we're using a FIT image as kernel (which includes kernel bin + DTB)
we enable the MTD_SPLIT_FIT_FW kernel option, which will detect the FIT
image and automatically split the "firmware" partition into 2 MTD parts
(kernel + rootfs).

The rootfs will then be parsed and split between rootfs + rootfs_data,
as usual.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44794 3c298f89-4303-0410-b956-a3cf2f4a3e73