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)
First cut at an HSI configuration API for rcc on the STMF4 series.

Fixed constant usage, and a AHB frequency computation.

And including an F0 rework of RCC a bit to put the rcc_set_hsi_clock code in
the F0 tree as well, tested on the Nucleo F030RE.

Further generalizing the clock setup to work for HSE and HSI clocks
F0 code updated to what should be the final architecture.
This commit adds changes to the l0 rcc system to be closer to the final
version.
This commit starts the process of add USART support to the l0 and pulls out the
usart common functions between l0 and f0 into a common file.
Fixed use of bare constants per BuFran's comments