ARM: sti: Provide DT nodes for SBC SSC[0..2]

The Synchronous Serial Controller is used to provide SPI.

These are the ports which are located on the Stand-By Controller (SBC).

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>