dt-bindings: arm,versatile-sysreg: Convert to DT schema format

Convert the arm,versatile-sysreg binding to DT schema format.

The original binding was missing 'simple-mfd' and a 'panel' sub node which
the only user (versatile-ab.dts) of this binding has.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220810160341.51995-3-robh@kernel.org