arm64: dts: st: add stm32mp257f-ev1 board support

Add STM32MP257F Evaluation board support. It embeds a STM32MP257FAI SoC,
with 4GB of DDR4, TSN switch (2+1 ports), 2*USB typeA, 1*USB2 typeC,
SNOR OctoSPI, mini PCIe, STPMIC2 for power distribution ...

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