core/include: Don't use 64 bit for MHZ & MiB macros
core/include: Don't use 64 bit for MHZ & MiB macros

Those macros are all about convenience. However, always using 64 bit makes casts
nececcary that goes against the idea of having a convenience macro.

E.g. when printing a frequency in KHZ one might want to do

	printf("freq: %lu kHz\n", freq / KHZ(1));

leads to an error

> error: format '%lu' expects argument of type 'long unsigned int', but argument 2 has type 'long long unsigned int'

Now we would have to cast - `%llu` is not available with newlib-nano and wholly
uneccecary.

Only use 64 bit artithmetic where necessary (GHZ, GiB), not for smaller units.

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