usb: dwc3: exynos: Make provision for vdd regulators

Facilitate getting required 3.3V and 1.0V VDD supply for
DWC3 controller on Exynos.

With patches for regulators' nodes merged in 3.15:
c8c253fc ARM: dts: Add regulator entries to smdk5420
275dcd22 ARM: dts: add max77686 pmic node for smdk5250,

certain perripherals will now need to ensure that,
they request VDD regulators in their drivers, and enable
them so as to make them working.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Cc: Anton Tikhomirov <av.tikhomirov@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>