Audit RTCPRE Shift and Mask Values

L0 - RTC Prescaler appears in RCC_CR, bit 20, 2 bits wide (RM0367 Rev 2)
L1 - RTC Prescaler appears in RCC_CR, bit 29, 2 bits wide (RM0038 Rev 10)
F0 - no prescaler setting (RM0091 Rev 7)
F1 - no prescaler setting (RM0041 Rev 4)
F2 - RTC Prescaler appears at RCC_CFGR, bit 16, 4 bits wide (RM0033 Rev 6)
F3 - no prescaler setting (RM0316 Rev 3)
F4 - RTC Prescaler appears at RCC_CFGR, bit 16, 4 bits wide (RM0090 Rev 8)