cpu/atmega: MCU implementation of the new GPIO API

[gschorcht]

Contribution description

This PR provides the MCU implementation of the low-level GPIO API in PR #14602 for ATmega. It also contains necessary changes for ATmega boards.

This PR depends on and includes PR #14602. Only the last two commits e461608 and 8aca4d1 are new in this PR.

Testing procedure

The following tests should be executed for any ATmega board:

• Flash tests/periph_gpio_exp and test whether basic GPIO functionality is still working.
• Use make -C tests/periph_gpio_exp test to test whether the redirection to virtual GPIO expanders works correctly.
• Since periph/pwm had to be changed, tests/periph_pwm should be used to check whether PWM is still working.
• Since periph/spi had to be changed, use any test that requires SPI to check whether SPI is still working.

Issues/PRs references

Depends on PR #14602

[gschorcht]

@maribu May I ask you to execute a short test this PR with drivers/wx281x. Since this driver includes a MCU specific part, I would like to know, whether the MCU implementation of the new low-level GPIO API works and maybe what the performance in comparison with the master for this driver is.

[maribu]

Actually, the old API was way too slow on the low end ATmega platform. Instead, a two hand-crafted assembly implementations doing the bit-banging are used - one working on 8 MHz ATmegas and one on 16 MHz ATmegas. The timing is done by relying that on ATmega each instruction takes 1 CPU cycle, except for branches and RAM accesses (which take 2).

So with the driver not using the GPIO API anyway (except for the pin setup), it won't be affected by any changes in the timing.

[gschorcht]

I see 😟 It would have been interesting to see the new API in action.

[gschorcht]

But drivers/ws281x should at least still work.

[stale]

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