Adds handling for missing cases. While i2c only has 3 cases, uarts have
all 4, so make sure they're handled properly.
Removes duplicated/redundant definitions.
Adds doxygen wrappers, even if only for internal use.
Fixes: e41ac6ea71 stm32: added peripheral clock get helpers for all stm32
Signed-of-by: Karl Palsson <karlp@tweak.au>
Allows for abstraction for code that's dependent on knowing the source clock
for a peripheral. Implemented a few core peripherals that tend to have clock
tree differences between platforms (USART, timers, I2C, SPI).
