switchtec: Add Global Fabric Manager Server (GFMS) event

Add a new event type that is newly exposed by recent firmware. The event
will never occur if the firmware is too old. If user space tries to use
this event in an older kernel, it will just get an EINVAL which is
perfectly acceptable in the existing user space code.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>