system.cgi: correctly translate CPU frequency

system.cgi: correctly translate CPU frequency

The CPU frequency diagram used the same "translation" as the CPU load,
which was confusing. This patch introduces a dedicated translation for
"CPU frequency", which makes things a little bit better but still does
not solve a Deppenleerzeichen ("CPU-Frequenz Diagramm") in the German
translation.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>