We don't need to read-modify-write the CTRLA register to disable
the UART.
The entire CTRLA register is re-written just a few lines below, so
we can just set it to 0 to disable the UART.
There is also no need to reset the UART since we re-write all config
registers in init.
If a write to a full tsrb is attempted with disabled interrupts
or in a interrupt then a deadlock will occure. To avoid this make
space in the ringbuffer by synchronously writing to uart.
The UART TX and TX lines on SAMD5x and SAML1x can be inverted.
However, the flags don't do exactly what one would expect.
See errata 2.18.5: SERCOM-UART: TXINV and RXINV Bits Reference:
> The TXINV and RXINV bits in the CTRLA register have inverted functionality.
>
> Workaround:
> In software interpret the TXINV bit as a functionality of RXINV, and conversely,
> interpret the RXINV bit as a functionality of TXINV.
Instead of hard-coding the peripheral clocks to CLOCK_CORECLOCK
introduce helper functions to return the frequency of the individual
GCLKs and use those for baud-rate calculations.
This requires the GCLK to be part of the peripheral's config struct.
While this is already the case for most peripherals, this also adds
it for those where it wasn't used before.
As it defaults to 0 (CLOCK_CORECLOCK) no change is to be expected.
In fractional mode, 3 bits are used to store the fractional part.
Therefore we must multiply / divide by 8 instead of 10 in order to
get the correct values.
This adds supoprt for the Atmel SAMD51 & SAME54 SoC.
The SAME5x/SAMD5x is a line of Cortex-M4F MCUs that share peripherals
with the samd2x Cortex-M0+ and saml1x Cortex-M23 parts.
