powerpc/time: Fix clockevent_decrementer initalisation for PR KVM

In the recent commit 8b78fdb0 ("powerpc/time: Use
clockevents_register_device(), fixing an issue with large
decrementer") we changed the way we initialise the decrementer
clockevent(s).

We no longer initialise the mult & shift values of
decrementer_clockevent itself.

This has the effect of breaking PR KVM, because it uses those values
in kvmppc_emulate_dec(). The symptom is guest kernels spin forever
mid-way through boot.

For now fix it by assigning back to decrementer_clockevent the mult
and shift values.

Fixes: 8b78fdb0

 ("powerpc/time: Use clockevents_register_device(), fixing an issue with large decrementer")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>