dt-bindings: xilinx: Remove desciption for 16550 uart

Documentation/devicetree/bindings/serial/8250.yaml already contains
description for mentined 3 properties that's why remove them from
xilinx.txt file.

Signed-off-by: Michal Simek <[e-mail]>
Link: https://lore.kernel.org/r/d90a[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

Documentation/devicetree/bindings/xilinx.txt

···
 		- compatible : should contain "xlnx,xps-hwicap-1.00.a" or
 				"xlnx,opb-hwicap-1.00.b".
 
-      vi) Xilinx Uart 16550
-
-      Xilinx UART 16550 devices are very similar to the NS16550 but with
-      different register spacing and an offset from the base address.
-
-      Required properties:
-       - clock-frequency : Frequency of the clock input
-       - reg-offset : A value of 3 is required
-       - reg-shift : A value of 2 is required
-
       vii) Xilinx USB Host controller
 
       The Xilinx USB host controller is EHCI compatible but with a different