dts: n6: Allow using axisram2 in chainloaded application

Make axisram2 which is used in fsbl mode available as well to
chainloaded application in order not to loose 1M of RAM

In order to avoid conflicts with bootloader, verify that code + ro data
of the loaded application won't go further than bootloader start address.
This is done with a linker assert.

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