dt-bindings: mfd: syscon: Require specific compatible also for simple-mfd

The syscon bindings require a device specific compatible, beside the
"syscon".  However schema counts "simple-mfd" as such, which allows
simple-mfd+syscon to sneak in.

Adjust the match to be sure simple-mfd also comes with a device specific
compatible.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220817142246.828762-5-krzysztof.kozlowski@linaro.org