dt-bindings: mips: Add missing mt7688a-soc compatible

This patch adds the "ralink,mt7688a-soc" compatible to the ralink DT
bindings documentation. This compatible is already used by some MIPS
boards (e.g. omega2p.dts) but not yet documented. It will also be used
by the upcoming "GARDENA smart Gateway" support.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: devicetree@vger.kernel.org
Cc: linux-mips@vger.kernel.org