Fix http://bugzilla.kernel.org/show_bug.cgi?id=5546

The cpu_khz global is not initialized and remains 0 if you boot with
clock=pit, even if the processor does have a TSC.  This may have bad
ramifications since the variable is used in various places scattered around
the kernel, though I didn't check them all to see if they can tolerate cpu_khz
= 0.  You can observe the problem by doing "cat /proc/cpuinfo"; the cpu MHz
line says 0.000.

The fix is trivial; call init_cpu_khz() from init_pit(), just as it's called
from the timers/timer_foo.c:init_foo() for other values of foo.

Cc: john stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>