ARM: DT: STiH407: Add RMII pinctrl support

This patch adds the RMII pinctrl support for the Synopsys
MAC on STiH407 SoCs.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>