powerpc: Move some calculations from xxx_calibrate_decr to time_init

Previously the individual xxx_calibrate_decr functions would each
print the timebase and cpu frequency and calculate several values
such as tb_to_us and tb_to_xs.  This moves those printks and
calculations into time_init just after the call to the platform's
calibrate_decr function.

Signed-off-by: Paul Mackerras <paulus@samba.org>