ar71xx: Add support for the WNDR3800CH

Apparently this is a fairly common refurb unit of the 3800 series now,
with merely the recognition string changed in the boot loader.

Signed-off-by: Dave Taht <dave.taht@bufferbloat.net>

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42031 3c298f89-4303-0410-b956-a3cf2f4a3e73