boards: st: stm32mp257f_dk_stm32mp257fxx_m33.dts: add I2C node

This commit adds the i2c8 node to the device tree for the
STM32MP257F-DK board. This node corresponds to the i2c exposed by the
GPIO expansion connector [1] and not used by the cortex A35.

[1]: https://www.st.com/resource/en/user_manual/um3385-discovery-kit-with-stm32mp257f-mpu-stmicroelectronics.pdf
    Table 24. GPIO connector pinout

Co-authored-by: Charles Wenger <charles.wenger@savoirfairelinux.com>
Signed-off-by: Tanguy Raufflet <tanguy.raufflet@savoirfairelinux.com>