ipq806x: enable NAND flash support in the kernel

Previous patch set backported the recently posted NAND flash driver to
3.18 and 4.1 kernel. This patch now enables it in the kernel config.

There is no change to the partition layout and init yet. But the NAND
flash can be seen in the mtd list on an AP148:

root@OpenWrt:/# cat /proc/mtd
dev:    size   erasesize  name
mtd0: 10000000 00020000 "qcom-nandc"
...

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46569 3c298f89-4303-0410-b956-a3cf2f4a3e73