boards: st: stm32N6 boards xspi-nor-flash DTS configuration

This PR defines the "st,stm32-xspi-nor" compatible Node
and the "st,stm32-xspi-psram" compatible Node
in conformance to the DTS specifications
Includes the size property (in Bits) of the external memory device

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