arm: axxia: ddr_retention: change the size of stack_var from 1024 to 512

If the size of array stack_var is 1024, it will trigger the build
warning 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 | }
      | ^

Since substracting 1K for 4 times from current stack pointer is equal to
substracting 512 for 8 times, let's change array size to 512 and
recursions to be 8.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
[Improve patch to limit "for loop" to 512 to avoid overflow]
Signed-off-by: Liwei Song <liwei.song@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>