ARM: STi: DT: Add STiH407 family tsin4 pinctrl configuration

tsin4 can only be configured for serial data transfer. However
depending on board design, two alternate pin configurations
are available. One in pin-controller-front0 and the other in
pin-controller-front1.

pinctrl_tsin4_serial_alt3 is brought out on B2120 reference
design as TSC on NIMA slot of the B2004A daughter board.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>