usb: musb: dsps: add dt support

Added device tree support for dsps musb glue driver and updated the
Documentation with device tree binding information.

Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Santhapuri, Damodar <damodar.santhapuri@ti.com>
Signed-off-by: Ravi Babu <ravibabu@ti.com>
[afzal@ti.com: use '-' instead of '_' for dt properties]
Signed-off-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>