Also adapt the defines to the documentation
- CPUs define up to 4 power modes (from zero, the lowest power mode,
to PM_NUM_MODES-1, the highest)
- >> there is an implicit extra idle mode (which has the number PM_NUM_MODES) <<
Previously on saml21 this would always generate pm_set(3) which is an illegal state.
Now pm_layered will correctly generate pm_set(2) for IDLE modes.
Idle power consumption dropped from 750µA to 368µA and wake-up from standby is also
possible. (Before it would just enter STANDBY again as the mode register was never
written with the illegal value.)
Creating an `exti_config` array for a new MCU manually is tedious and error prone.
Luckiely all information is already availiable in the vendor files.
Credit for this discovery & method goes to @Sizurka
The file was generated with
```C
int main(void) {
puts("static const int8_t exti_config[PORT_GROUPS][32] = {");
for (unsigned port = 1; port < 5; ++port) {
printf("#if PORT_GROUPS >= %d\n{\n", port);
for (unsigned pin = 0; pin < 32; ++pin) {
printf("#ifdef PIN_P%c%02uA_EIC_EXTINT_NUM\n", '@' + port, pin);
printf(" PIN_P%c%02uA_EIC_EXTINT_NUM,\n", '@' + port, pin);
printf("#else\n -1,\n#endif\n");
}
printf("},\n#endif\n\n");
}
puts("};");
return 0;
}
```
No changes in generated code are expected, but this makes adding new members
of the sam0 CPU families much easier.
