`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.
There are some constraints to the oscillators on the samd5x.
- DFLL is fixed to run at 48 MHz
- DPLL can run at 96 to 200 MHz
Always use DFLL for frequencies <= 48 MHz.
For frequencies >= 96 MHz, use DPLL directly.
For frequencies < 96 MHz, clock DPLL at twice the desired frequency
and use a divider.
- Removed stdio_init() from newlib's _init(), as this is too late in the boot
process to allow DEBUG()ing during periph_init()
- Added stdio_init() to the various cpu_init() routines of the ARM CPUs just
before periph_init()
There were still some things wrong with samd5x CPU init which only
showed up when used in conjunction with RIOTBOOT, that is cpu_init()
was called twice.
- gclk_connect() should block until the GCLK is ready.
- DPLL should be disabled dring configuration.
- make sure not to use DPLL for MCLK when re-configuring DPLL
- All APBxMASK bits should be in a defined state.
- always enable 1kHz oscilator output.
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.
