dt-bindings: arm: stm32: Add Phytec STM32MP1 board

The Phytec STM32MP1 based SoMs feature up to 1 GB DDR3LP RAM, up to
1 GB eMMC, up to 16 MB QSPI and up to 128 GB NAND flash.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>