boards: st: nucleo_l552ze_q: correct ns variant RAM info

TF-M owns the last 64kByte of SoC internal SRAM1 in 'ns' variant of
board nucleo_l552ze_q. Update the board variant DTS and YAML files
accordingly and remove the default allocation of 2kB of malloc pool
that worked around this missing information.

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