dt-bindings: ufs: mediatek,ufs: Document optional dvfsrc/va09 regulators

Document the optional dvfsrc-vcore and va09 regulators used for,
respectively, crypt boost and internal MPHY power management in
when powering on/off the (external) MediaTek UFS PHY.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>