ralink: set ralink_soc on mt7621

sdhci failed to init because of this

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43293 3c298f89-4303-0410-b956-a3cf2f4a3e73