powerpc/kdump: Delay before sending IPI on a system reset

If we enter the kdump code via system reset, wait a bit before
sending the IPI to capture all secondary CPUs. Without it we race
with the hypervisor that is issuing the system reset to each CPU.
If the IPI gets there first the system reset oops output then shows
the register state of the IPI handler which is not what we want.

I took the opportunity to add defines for all the various delays
we have. There's no need for cpu_relax when we are doing an mdelay,
so remove them too.

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