powerpc/eeh: Export eeh_unfreeze_pe()

The PowerNV hotplug driver needs to be able to clear any frozen PE(s)
on the PHB after suprise removal of a downstream device.

Export the eeh_unfreeze_pe() symbol to allow implementation of this
functionality in the php_nv module.

Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/1778535414.1359858.1752615454618.JavaMail.zimbra@raptorengineeringinc.com