MIPS: ralink: Add missing pinmux.

The mt7620 has a pin that can be used to generate an external reference
clock. The pinmux setup was missing the definition of said pin. This patch
adds it.

Signed-off-by: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14898/


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