stm32mp257f_ev1_stm32mp257fxx_m33.dts: add spi node

This commit adds the spi3 node to the device tree for the
STM32MP257F-EV1 board. This node corresponds to the spi exposed by the
GPIO expansion connector [1].

[1]: https://wiki.st.com/stm32mpu/wiki/STM32MP257x-EV1_-_hardware_description



Signed-off-by: Tanguy Raufflet <tanguy.raufflet@savoirfairelinux.com>