ARM: dts: STiH407: Add B2120 board support

B2120 HDK is the reference board for STiH407 SoC.
It has the following characteristics:
 - 1GB DDR3
 - 8GB eMMC / SD-Card slot
 - 32MB NOR Flash
 - 1 x Gbit Ethernet
 - 1 x USB 3.0 port
 - 1 x Mini-PCIe
 - 1 x SATA
 - 1 x HDMI output
 - 1 x HDMI input
 - 1 x SPDIF

This patch only introduces basic functionnalities, such as I2C and UART.

Acked-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: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>