AA: ramips: add support for DIR-620D1

Both DIR-615H1 and DIR-620D1 are ralink rt3352 based router. But DIR-620D1 have 8MB flash and USB port.

Signed-off-by: Serge Vasilugin <vasilugin@yandex.ru>

git-svn-id: svn://svn.openwrt.org/openwrt@36557 3c298f89-4303-0410-b956-a3cf2f4a3e73