brcm63xx: Add DGND3700 v1 / DGND3800B support

This adds support for the Netgear DGND3700 v1 and DGND3800B.

DGND3700 v1 and DGND3800B are exactly the same router, only one difference,
the factory firmware. This patch produces two factory firmwares for each router.

The router includes an onboard NAND flash for extra storage. NAND is currently
not supported in bcm63xx, so no code added for this part. The NOR flash (32MB)
is enough for storing an openwrt firmware.

Signed-off-by: Daniel Gonzalez <dgcbueu@gmail.com>
[jogo: Add back commit message from v1]
Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44831 3c298f89-4303-0410-b956-a3cf2f4a3e73