RAM retention not working on NXP S32K3

[manuargue]

Describe the bug
S32K3 devices allow to retain SRAM contents through a hw mechanism but currently the SRAM contents are always being zero-initialized at boot time to prevent ECC errors for occurring: https://github.com/zephyrproject-rtos/zephyr/blob/main/soc/nxp/s32/s32k3/s32k3xx_startup.S

To Reproduce

1. write data in SRAM after booting the device
2. issue a functional reset (e.g. watchdog expires - not currently supported in main branch)
3. read previous data from SRAM after booting the device - contents are cleared.

Expected behavior
SRAM contents are retained across functional resets and ECC initialization is only performed if coming from a destructive reset (e.g. PoR).

Impact
defective functionality

Environment (please complete the following information):

• OS: Linux
• Toolchain Zephyr SDK
• d2a4522

Additional context
NA