boards: st: Fix arduino_gpio support

On stm32h735g_disco, `arduino_gpio` is declared while the connector is not
actually described.
On stm32h750b_dk, connector description was simply not included.

All these are blocking tests trying to make profit of arduino gpio
connectors.
Fix these.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>