boards: st: b_u585i_iot02a: Extract external partitions

The S and NS shares the extermal NOR memory. This extracts the external
NOR partition definitions from b_u585i_iot02a-common.dtsi and move to
b_u585i_iot02a.dts to allow NS software defined their own external
partition layout.

Signed-off-by: BUDKE Gerson Fernando <gerson.budke@leica-geosystems.com>