usb: dwc: allow pre-configuration of DCTL[HIRD_Thresh]

Allows the user to set a value for DCTL[HIRD_Thresh] (which controls
the UTMI sleep mechanism vs. the PHY) in the DT.
This goes towards two benefits:
1. Fixes a long-standing TODO in dwc3/gadget.c (originally
introduced in 1a947746)
2. Allows compliance to the HPG for Link Power Management on
mdm9630

Change-Id: Id6aab40836de29994d1dcb6344efa5fae56724d1
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>