ARM: dts: stm32: add initial support of stm32mp157c-dk2 board

Add support of stm32mp157c discovery2 board (part number: STM32MP157C-DK2).
This board is a "super-set" of stm32mp157a-dk1. It embeds a STM32MP157c SOC
with AC package (TFBGA361, 148 ios) and 512MB of DDR3. Same connections
than stm32mp157a-dk1 board are available. Display panel (otm8009a) and
Murata wifi/BT combo is added.

This patch adds basic support for a kernel boot and enable otm8009a display
panel.

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