noinit region of RAM to store data between reboots

I'm implementing a feature which requires me to store data in ram region which would not be resetted during reboots (sys_reboot(SYS_REBOOT_WARM)). 

I'm using clause in application :

__attribute__((section(".noinit"))) static volatile uint32_t EXPERIMENTAL_counter;
which does not work, as EXPERIMENTAL_counter does not retain value between reboots.
System uses NRF53, Zephyr and MCUboot prior to boot of Appcore. My diagnosis is that MCUboot messes up this .noinit area from appcore. 
Should I create a separate region in partition_manager.yml? If yes, how it should look like? 
What is recommended way to solve data retention in RAM between warm reboots (with bootloader)? 
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