dt-bindings: usb: exynos-usb: Add missing required VDD properties

Since commit bd8ce544 ("usb: dwc3: exynos: Make provision for vdd
regulators") vdd33-supply and vdd10-supply are required so document them
in bindings.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>