MIPS: ralink: Fix vendor string for mt7620

Ralink was acquired by Mediatek. Represent this in the cpuinfo. It
apparently confused people.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11994/


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