Documentation: of-serial.txt - update for clocks phandle for clk

Signed-off-by: NMurali Karicheri <m-karicheri2@ti.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/tty/serial/of-serial.txt

@@ -14,7 +14,10 @@ Required properties:
 	- "serial" if the port type is unknown.
 - reg : offset and length of the register set for the device.
 - interrupts : should contain uart interrupt.
-- clock-frequency : the input clock frequency for the UART.
+- clock-frequency : the input clock frequency for the UART
+	 or
+  clocks phandle to refer to the clk used as per Documentation/devicetree
+  /bindings/clock/clock-bindings.txt
 
 Optional properties:
 - current-speed : the current active speed of the UART.