boards: st: stm32mp135f_dk: add the I/O expander i2c device and led 2-3

The stm32mp135f_dk board embeds an I/O expander to get access
to more I/O. Led 3 (green) and Led 4 (orange) are accessible via this
I/O expander.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>