ARM: dts: stm32: add initial support of stm32mp157a-dk1 board

Add support of stm32mp157a discovery1 board (part number: STM32MP157A-DK1).
This board embeds a STM32MP157a SOC with AC package (TFBGA361, 148 ios)
and 512MB of DDR3. Several connections are available on this boards:
4*USB2.0, 1*USB2.0 typeC, SDcard, RJ45, HDMI, Arduino connector, ...

This patch enables basic support for a kernel boot.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>