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 <fabio.estevam@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>