usb: phy-msm-qusb-v2: Add support to control external pull down

Add phy driver call back to enable external pull down on D+
line in host mode. External pull-down is only needed during
port reset before enabling high speed terminations. Pull-down
will be disabled 20ms after its enabled in timer function.

Change-Id: Id2a4e34b4937dc63d3e20cd48f5d586bbcb7599a
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>