PPC: Add clock-frequency export for Mac machines

Support in fwcfg has been around for exposure of the clock-frequency
CPU property. OpenBIOS reads it, we just never exposed it.

Since Mac OS X is very picky about its clock frequency values, let's
just take a known good value and always expose that.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Alexander Graf <agraf@suse.de>
