mfd: omap-usb-tll: Update DT clock binding information

The omap-usb-tll driver needs one clock for each TLL channel.
Add this information to the DT binding document.

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>