dt-bindings: serial: omap_serial: add clocks entry

Document clocks property used to pass phandle to functional clk.
Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/omap_serial.txt

@@ -22,6 +22,8 @@ Optional properties:
 - dma-names : "rx" for receive channel, "tx" for transmit channel.
 - rs485-rts-delay, rs485-rx-during-tx, linux,rs485-enabled-at-boot-time: see rs485.txt
 - rs485-rts-active-high: drive RTS high when sending (default is low).
+- clocks: phandle to the functional clock as per
+  Documentation/devicetree/bindings/clock/clock-bindings.txt
 
 Example: