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 <[e-mail]>
Signed-off-by: Roger Quadros <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/mfd/omap-usb-tll.txt

···
 - interrupts : should contain the TLL module's interrupt
 - ti,hwmod : must contain "usb_tll_hs"
 
+Optional properties:
+
+- clocks: a list of phandles and clock-specifier pairs, one for each entry in
+  clock-names.
+
+- clock-names: should include:
+  * "usb_tll_hs_usb_ch0_clk" - USB TLL channel 0 clock
+  * "usb_tll_hs_usb_ch1_clk" - USB TLL channel 1 clock
+  * "usb_tll_hs_usb_ch2_clk" - USB TLL channel 2 clock
+
 Example:
 
 	usbhstll: usbhstll@4a062000 {