cpu/sam0_common: GPIO always default to MAIN clock for EXTI, make configurable

[benpicco]

Contribution description

Currently only samd21 used the 32 kHz clock for EXTI which makes it miss fast events.
All newer members of the family use the MAIN clock.

While touching this, also make the clock source configurable to this can be overwritten, e.g. in the board config if desired.

Testing procedure

GPIO interrupts should still work.

Issues/PRs references

[fjmolinas]

It seems that using the 32Khz clock was introduced in #6160, seems like it was done to have the GPIO working in low power modes, is this something that is arleady handled in gpio_pm_cb_enter?

[benpicco]

is this something that is arleady handled in gpio_pm_cb_enter?

Yes gpio_pm_cb_enter will configure the 32kHz clock and gpio_pm_cb_leave will restore the main clock.
Actually it already does this now, so when waking from resume the EXTI peripheral will already run off the main clock in master.

[fjmolinas]

ACK, GPIO irq stil work on samr21-xpro.

2020-06-23 22:47:46,204 # main(): This is RIOT! (Version: 2020.07-devel-1284-gc0203a-pr-14320)
2020-06-23 22:47:46,210 # This application allows you to test the CPU power management.
2020-06-23 22:47:46,215 # The available power modes are 0 - 2. Lower-numbered power modes
2020-06-23 22:47:46,221 # save more power, but may require an event/interrupt to wake up
2020-06-23 22:47:46,224 # the CPU. Reset the CPU if needed.
2020-06-23 22:47:46,226 # mode 0 blockers: 1 
2020-06-23 22:47:46,228 # mode 1 blockers: 0 
2020-06-23 22:47:46,229 # mode 2 blockers: 0 
2020-06-23 22:47:46,231 # Lowest allowed mode: 1
2020-06-23 22:47:46,234 # using BTN0 as wake-up source
> 2020-06-23 22:47:46,495 #  BTN0 pressed.
2020-06-23 22:47:46,496 # BTN0 pressed.
2020-06-23 22:47:46,638 # BTN0 pressed.
2020-06-23 22:47:46,639 # BTN0 pressed.
2020-06-23 22:47:47,270 # BTN0 pressed.
2020-06-23 22:47:47,271 # BTN0 pressed.
2020-06-23 22:47:47,388 # BTN0 pressed.
2020-06-23 22:47:47,505 # BTN0 pressed.
2020-06-23 22:47:47,616 # BTN0 pressed.
2020-06-23 22:47:48,387 # BTN0 pressed.
2020-06-23 22:47:48,388 # BTN0 pressed.
2020-06-23 22:47:49,560 # BTN0 pressed.
2020-06-23 22:47:49,561 # BTN0 pressed.
2020-06-23 22:4750,241 # BTN0 pressed.