phy-msm-qusb-v2: Add support for enabling external pull down for USB2 port

Add support for enabling external pull down for USB2 port so that
USB HS devices detection works without any issues.

Change-Id: Id5ac67bfbcb7b2485e59e31c594fe9462e95079c
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>