stm32cube: stm32n6: Allow building a chainloaded secure application

When building a chain-loaded secure application, build and call
system_stm32n6_s.c file.
Similarly to what is done on system_stm32n6xx_fsbl.c, remove CMSE_NS_ENTRY
modifier as Secure FW has no entry funcs.

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