PCI: microchip: Make the struct event_descs static

PCI: microchip: Make the struct event_descs static

The struct event_descs does not have any users outside the
pcie-microchip-host.c file, and has no previous declaration,
thus it can be made static.

This resolves the following sparse warning:

  drivers/pci/controller/pcie-microchip-host.c:352:3: warning: symbol 'event_descs' was not declared. Should it be static?

Link: https://lore.kernel.org/r/20210509041932.560340-1-kw@linux.com
Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>