powerpc: Fix some bugs in the new merged time code

I had the sense of the test for when to use the old 601-style RTC
registers inverted.  pmac_calibrate_decr and via_calibrate_decr
weren't setting ppc_tb_freq, on which all the further calculations
depended.  Lastly, update_gtod was losing the top 32 bits of
the new tb_to_xs value.

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