dt-bindings: clock: mediatek,mtmips-sysc: Adapt compatible for MT7688 boards

dt-bindings: clock: mediatek,mtmips-sysc: Adapt compatible for MT7688 boards

As the MT7628 and MT7688 are identical in most respects, mt7628a.dtsi is
used for both SoCs. To prevent "Kernel panic - not syncing: unable to
get CPU clock, err=-2" and allow an MT7688-based board to boot, the
following must be allowed:

    compatible = "ralink,mt7628-sysc", "ralink,mt7688-sysc", "syscon";

Signed-off-by: Ezra Buehler <ezra.buehler@husqvarnagroup.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>