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

This is to fix compile warning which is introduced by the SDK
commit a4396719ad21 ("ARM: axxia: Fix support for kernel 5.10")
as below:

arch/arm/mach-axxia/ddr_retention.c:220:1: warning: the frame size of 1032 bytes is larger than 1024 bytes [-Wframe-larger-than=]
  220 | }
      | ^

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