MAINTAINERS: Update PCI Cadence maintainer entry

Replace Alan Douglas with Tom Joseph as the current PCI
Cadence host/endpoint controller maintainer.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Tom Joseph <tjoseph@cadence.com>