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.
