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: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>