Since https://github.com/RIOT-OS/RIOT/pull/20935 gpio_write()
uses a `bool` instead of an `int`. This does the same treatment for
`gpio_read()`.
This does indeed add an instruction to `gpio_read()` implementations.
However, users caring about an instruction more are better served with
`gpio_ll_read()` anyway. And `gpio_read() == 1` is often seen in
newcomer's code, which would now work as expected.
The EFM32 uses the provided _SILICON_LABS_32B_SERIES_0 and
_SILICON_LABS_32B_SERIES_1 definitions to enable or disable certain
code. With the introduction of new MCUs, there is also the
_SILICON_LABS_32B_SERIES_2 definition.
This PR ensures that the defines are explicit, and that #else
statements don't target the wrong series.
