powerpc/powernv: Add ppc_pci_reset_phbs parameter to issue a PHB reset

During a kdump kernel boot in PowerPC, we request a reset of the PHBs
to the FW. It makes sense, since if we are booting a kdump kernel it
means we had some trouble before and we cannot rely in the adapters'
health; they could be in a bad state, hence the reset is needed.

But this reset is useful not only in kdump - there are situations,
specially when debugging drivers, that we could break an adapter in
a way it requires such reset. One can tell to just go ahead and
reboot the machine, but happens that many times doing kexec is much
faster, and so preferable than a full power cycle.

This patch adds the ppc_pci_reset_phbs parameter to perform such reset.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>