ARM: dts: msm: allow pre-configuration of DCTL[HIRD_Thresh]

Allows the user to set a value for DCTL[HIRD_Thresh] which, in turn,
controls the UTMI sleep mechanism vs. the PHY. 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: Ic804fe7a4b68170dbde917171e520a7b11f6c490
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>