dwc_otg: Support switch host and device directly via sysfs.

dwc_otg: Support switch host and device directly via sysfs.

BZ: 31708

Currently, the sysfs interface can't support switch HOST to DEVICE or
DEVICE to HOST directly. We have to restart whole HVP platform to do
this switch. There are two patches to fix this issue, this patch is for
host side fixing.

Fix build warning, and merge part of code about vbus control from Synopsys
reference code.

Signed-off-by: Yu Wang <yu.y.wang@intel.com>
Change-Id: Ic6ddb01174a8990b833d0f2b27a8a7447d69b7e2
Signed-off-by: Yu Wang <yu.y.wang@intel.com>
Reviewed-on: http://android.intel.com:8080/43423
Tested-by: Sun, Jianhua <jianhua.sun@intel.com>
Reviewed-by: Tang, Richard <richard.tang@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>