MAINTAINERS: Add missing /drivers/pci/cadence directory entry

Commit 37dddf14 ("PCI: cadence: Add EndPoint Controller driver for
Cadence PCIe controller") created the /drivers/pci/cadence directory to
keep in a single place Cadence host and endpoint controller drivers.

Since code in /drivers/pci/cadence falls within the PCI native host
bridge and endpoint controllers mainteinance remit, that maintainer
entry should have been updated too by adding the /drivers/pci/cadence
directory to it but it actually was not.

Update the MAINTAINERS entry accordingly, fixing the omission.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Alan Douglas <adouglas@cadence.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>