Allow semi-colon list in frequency=*

[midgard]

Currently frequency=* is validated against ^{(0|[1-9][0-9]*(\.[0-9]+)?)( (kHz|MHz|GHz|THz))?$ (in resources/data/validator/numeric.mapcss). Semi-colon separated lists are also valid, for power lines with multiple circuits at different frequencies. This is common when power lines for the railways are piggy-backing on transmission power lines, e.g. at ​https://www.openstreetmap.org/way/208904880.}

Expected: assertNoMatch: "way frequency=50;50;16.7";

Actual: "way frequency=50;50;16.7" matches and gives validator warning