ppc: fix ciface_milliseconds using incorrect frequency for delay

Instead of using a constant frequency for all CPUs, use the processor timebase
frequency provided by QEMU. This fixes issues where delays from
ciface_milliseconds were not corresponding to elapsed time as given by the real
time clock on certain platforms, eg. QEMU's mac99 machine.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>