[POWERPC] Modify sched_clock() to make CONFIG_PRINTK_TIME more sane

When booting a current kernel with CONFIG_PRINTK_TIME enabled you'll
see messages like:

[    0.000000] time_init: decrementer frequency = 188.044000 MHz
[    0.000000] time_init: processor frequency   = 1504.352000 MHz
[3712914.436297] Console: colour dummy device 80x25

This cause by the initialisation of tb_to_ns_scale in time_init(), suddenly the
multiplication in sched_clock() now does something :).  This patch modifies
sched_clock() to report the offset since the machine booted so the same
printk's now look like:

[    0.000000] time_init: decrementer frequency = 188.044000 MHz
[    0.000000] time_init: processor frequency   = 1504.352000 MHz
[    0.000135] Console: colour dummy device 80x25

Effectivly including the uptime in printk()s.

This patch makes tb_to_ns_scale and tb_to_ns_shift static and
read_mostly for good measure.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>