dt-bindings: mfd: bd9571mwv: Document DDR Backup Mode properties

Document the new optional properties related to DDR Backup Mode and
toggle/momentary power switches.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>