boards: st: b_u585i_iot02a: correct ns variant RAM info

TF-M owns the last 64kByte of SoC internal SRAM1 in 'ns' variant of
board b_u585i_iot02a. Update the board variant DTS and YAML files
accordingly and use RAM3 (512kB) as default Zephyr RAM since it is
larger than the remaining 128kB left in SRAM1.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
2 files changed