MIPS: ralink: Add support for mt7688

MT7688 is similar tot he MT7628 but has a different wifi radio.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11439/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>