The registers RCM->SSRS0 and RCM->SSRS1 are listed in the SDK file MK22F51212.h. There is no documentation the K22 sub-family referenced manual, rev 5, 2015.
The RCM documentation section stops at the Mode register, MR.
Where would I find documentation on SSRS0 and SSRS1?
This does appear to be a set of registers that may or may not be able as the SDK has a flag in MK22F51212_features.h
/* @brief Has sticky system reset status register RCM_SSRS0 and RCM_SSRS1. */
#define FSL_FEATURE_RCM_HAS_SSRS (1)