ARM: dts: msm: Fix host mode issue on sdx20

The change 'commit 3a07a4e7 ("ARM: dts: msm: Delete
redundant USB nodes for SDX20")' indirectly deleted
the vbus_dwc3-supply attribure as part of deleting the
redundant nodes. This caused a regression in the host
mode of sdx20. Fix this by adding the vbus_dwc3-supply back.

Change-Id: Icefb90f5be6b35a9ff07964400f7ac7956b8a613
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>