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.
Currently, spi_acquire() will always re-configure the SPI bus.
If the configuration did not change, this is entirely uneccecary
and makes SPI operations take longer than needed.
Instead, compare the current configuration with the new configuration
and skip the initialisation if it didn't change since the last call.
