ARM: dts: STiH407-family: Use new Pinctrl groups

Having just defined some new Pinctrl groups for when HW flow-control
is {en,dis}abled, let's reference them for use within the driver.

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>