Set musb config->maximum_speed based on the dts setting to control musb
speed.

By default musb works in high-speed mode. Adding

	maximum-speed = "full-speed";

to dts usb node will force musb to full-speed mode.
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>