dt-bindings: serial: Update 'uart-has-rtscts' description

'uart-has-rtscts' property and 'rts-gpios|cts-gpios' are normally
mutually exclusive, however it is possible for some drivers to have
a dynamic approach, meaning that both properties can be relevant.
Acked-by: NPeter Griffin <peter.griffin@linaro.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/serial.txt

@@ -23,7 +23,8 @@ Optional properties:
     they are available for use (wired and enabled by pinmux configuration).
     This depends on both the UART hardware and the board wiring.
     Note that this property is mutually-exclusive with "cts-gpios" and
-    "rts-gpios" above.
+    "rts-gpios" above, unless support is provided to switch between modes
+    dynamically.
 
 
 Examples: