Cyclon V Reset manager status register always read 0!

In our project we have u-boot v2018.11. I have been requested to write a functionality related to reset reason. Following the already implemented functionality mainly in iMX devices I wrote similar function. And read values from the register is always 0! Why? I thought it could be related toincorrect access to the register. In Cyclon V HPS TRM is written that:

Important:To prevent indeterminate system behavior, reserved areas of memory must not be accessed bysoftware or hardware. Any area of the memory map that is not explicitly defined as a registerspace or accessible memory is considered reserved

Any suggestions?