usb: bam: Define usb_bam as child of USB controller device

As usb_bam driver now supports separate DT nodes for each
core, define usb-bam DT node under its USB core.
This allows usb_bam device to directly access its parent
device and remove msm_bam_set_usb_dev API. Having usb_bam
as child will allow it to maintain power management using
its own device instead of invoking PM calls on parent
directly.

Change-Id: I5ec28be10005b364159672aaa00544584928da1e
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>