Hello Peter,
When using SYSCTL_RCC_R you do not need to use HWREG. If you look at the definition it already does the HWREG macro. Your code should like
SYSCTL_RCC_R |= SYSCTL_RCC_USEPWMDIV;
These XY_R macros are defined in the inc/tm4c123gh6zrb.h
Regards,
Amit