phy: exynos-usb2: add vbus regulator support

Exynos USB2 PHY has separate power supply, which is usually provided by
VBUS regulator. This patch adds support for it. VBUS regulator is
optional, to keep compatibility with boards, which have VBUS provided
from some always-on power source.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>