powerpc/pseries: Advertise Hot Plug Event support to firmware

With the inclusion of commit 333f7b76 ("powerpc/pseries: Implement
indexed-count hotplug memory add") and commit 75384347


("powerpc/pseries: Implement indexed-count hotplug memory remove"), we
now have complete handling of the RTAS hotplug event format as described
by PAPR via ACR "PAPR Changes for Hotplug RTAS Events".

This capability is indicated by byte 6, bit 2 (5 in IBM numbering) of
architecture option vector 5, and allows for greater control over
cpu/memory/pci hot plug/unplug operations.

Existing pseries kernels will utilize this capability based on the
existence of the /event-sources/hot-plug-events DT property, so we
only need to advertise it via CAS and do not need a corresponding
FW_FEATURE_* value to test for.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>