cuda: rename frequency property to tb_frequency

This allows us to more easily differentiate between the timebase frequency used
to calibrate the MacOS timers and the actual frequency of the hardware clock as
indicated by CUDA_TIMER_FREQ.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[dwg: Revert some extraneous changes which break compile]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>