serial: fsl-imx-uart.txt: Remove generic property

'uart-has-rtscts' is a generic serial property and it is described
at Documentation/devicetree/bindings/serial/serial.txt, so remove it
from the specific fsl-imx-uart binding documentation.

While at it, add a note pointing to the serial.txt file, which
contains the complete list of generic serial bindings.

Signed-off-by: Fabio Estevam <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/serial/fsl-imx-uart.txt

···
 - interrupts : Should contain uart interrupt
 
 Optional properties:
-- uart-has-rtscts : Indicate the uart has rts and cts
 - fsl,irda-mode : Indicate the uart supports irda mode
 - fsl,dte-mode : Indicate the uart works in DTE mode. The uart works
                   in DCE mode by default.
+
+Please check Documentation/devicetree/bindings/serial/serial.txt
+for the complete list of generic properties.
 
 Note: Each uart controller should have an alias correctly numbered
 in "aliases" node.