- rcc_get_i2c_clk_freq: Add support by taking RCC_CCIPR2 into account for I2C4.
- rcc_get_timer_clk_freq: Add support for LP timers
- rcc_get_usart_clk_freq: Fix APB freq used for LPUART1 and UART1, respectively.
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).
rcc_osc_bypass_enable and rcc_osc_bypass_disable have been copy/pasted
around for the last time! There's a compile bit to check for L0/L1, but
otherwise this is just code duplication for no gain.
