soc: s32k3: fix RAM retention

Initialize TCM and SRAM contents only after a destructive reset (e.g.
PoR reset). SRAM retains content during functional reset through a
hardware mechanism, therefore accesses do not cause content
corruption errors.

Fixes #75912

Signed-off-by: Manuel Argüelles <marguelles.dev@gmail.com>
(cherry picked from commit ec728903)