ARM: dts: stm32: add initial support of stm32mp157c eval board

ARM: dts: stm32: add initial support of stm32mp157c eval board

Add support of stm32mp157c evaluation board (part number: STM32MP157C-EV1)
split in 2 elements:
-Daughter board (part number: STM32MP157C-ED1)
 which includes CPU, memory and power supply
-Mother board (part number: STM32MP157C-EM1)
 which includes external peripherals (like display, camera,...)
 and extension connectors.

The daughter board can run alone, this is why the device tree files
are split in two layers, for the complete evaluation board (ev1)
and for the daughter board alone (ed1).

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>