powerpc/time: Use clocksource_register_hz

Use clocksource_register_hz which calculates the shift/mult
factors for us. Also remove the shift = 22 assumption in
vsyscall_update - thanks to Paul Mackerras and John Stultz for
catching that.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>