ARM: dts: stm32: add initial support for stm32mp157-odyssey board

Add support for Seeed Studio's stm32mp157c odyssey board.
Board consists of SoM with stm32mp157c with 4GB eMMC and 512 MB DDR3 RAM
and carrier board with USB and ETH interfaces, SD card connector,
wifi and BT chip AP6236.

In this patch only basic kernel boot is supported and interfacing
SD card and on-board eMMC.

Signed-off-by: Marcin Sloniewski <marcin.sloniewski@gmail.com>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>