boards: stm32n6570_dk: Review variants organization

Add a new /fsbl variant which replaces the default variant and typically
fits for development or bootloader (MCUBoot typical target). Image
is limited to 511KB.

Default variant now stands for the real application which is supposed
to be loaded by the bootloader. It runs from axisram1 and could be both
XIP or RAMLOADED. Image has access to the whole AXISRAM1 and 2 (2MB).

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