usb: dwc3: exynos5: Power-off VBUS during suspend.

usb: dwc3: exynos5: Power-off VBUS during suspend.

Change de-asserts vbus_gpio on suspend and re-asserts on resume in
order to save power.  NOTE this change means no USB charging in
standby is supported.

BUG=chrome-os-partner:9156
TEST=manual
1. Boot device, login guest, ctrl-alt-t for shell
2. shell
3. sudo su
4. Measure USB3.0 VBUS is 5V
5. powerd_suspend
6. Measure USB3.0 VBUS is off
7. resume
8. Measure USB3.0 VBUS is 5V

~30mW of savings in suspend.

Change-Id: Iaeaadde8b041d731c5114ab7318e4b8d8caebef9
Signed-off-by: Todd Broch <tbroch@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/33642
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
Commit-Ready: Doug Anderson <dianders@chromium.org>