Setting up a DMA transfer can take longer than sending out a buffer
byte by byte if the buffer is small.
DMA only shows advantages for large buffers, using it for every transfer
will cause a net slowdown.
Since we did not come up with a good way to determine the treshold based
on the SPI frequency, just use a fixed buffer for now so that DMA can be
used without slowing things down overall.
`pm_set()` gets called by the idle thread whose stack is too small
for normal DEBUG()/printf().
Use DEBUG_PUTS() instead to print the static debug strings.
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.
