IO peripheral baud rate selection

[prattmic]

There should be a way to specify the baud rate of IO peripherals (UART, I2C, SPI, etc), preferably at runtime, and in standard units.

Currently, on the STM32F4, the UART is hard coded to 115200, and the SPI is hard coded to PCLK/8. As a result, the actual clock rate depends on which SPI peripheral is used (different peripherals have different PCLK values), and the system clock speed.