boards: st: stm32l5: Fix TF-M by restricting Libc malloc aera

By default, libc malloc allocated area is using all available RAM.
For some yet unknown reason, this conflicts with TF-M resulting in a
Hard Fault before jumping in the non secure application.

For now, define a Libc malloc area defined to 2048 which is the default in
some other typical applications (ARMv7 targets enabling USERSPACE).

Fixes #77847

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