usb: bam: Separate out USB BAM device tree documentation

Since the USB BAM is represented as its own device in device tree,
as well as a unique driver apart from the various USB controllers,
it doesn't make sense to keep it documented in "msm-hsusb.txt",
thus it deserves its own file.

Change-Id: Ib118a9f0af1fe49f8688710515bea4d646a21be3
Signed-off-by: Jack Pham <jackp@codeaurora.org>