usb: dwc3: exynos: add basic suspend/resume support

Adds suspend and resume callbacks to exynos dwc3 driver as part of
power management support.

This change does gating of dwc3 clock during suspend/resume cycles.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Vikas C Sajjan <vikas.sajjan@linaro.org>
CC: Doug Anderson <dianders@chromium.org>
Tested-by: Vivek Gautam <gautam.vivek@samsung.com>

[ balbi@ti.com : refreshed to current linus/master ]

Signed-off-by: Felipe Balbi <balbi@ti.com>