boards: arm: stm32 disco kit has ospi nor flash node

Define the reg and size property for the stm32 disco kits
which have an octospi instance
Refer to the dts/bindings/flash_controller/st,stm32-ospi-nor.yaml.
Also remove the <size> property for the stm362h750 disco kit.

Signed-off-by: Francois Ramu <francois.ramu@st.com>