Revert "arm: axxia: ddr_retention: use malloc/free to fix -Wframe-larger-than build warning"

This reverts commit b48ce4a4.

The intent of exercise_stack_ptr is to keep the 4K-size stack of
ncp_ddr_shutdown in L2 cache for the later usage.

But using heap will break this. So revert it now.

Reported-by: Mikael Beckius <mikael.beckius@windriver.com>
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>