Revert "soc: nordic: s2ram: Align s2ram marking procedures"

The marking procedures can be reverted, since at this stage
it is already allowed to use the stack.
The MSP is temporairly set to interrupt stack in `reset.S` file before
calling s2ram procedures..

Signed-off-by: Adam Kondraciuk <adam.kondraciuk@nordicsemi.no>

This reverts commit e68d110f7c175f2a9275659d023c9d37d1e4b4f1.