MAINTAINERS: clean up PCIE DRIVER FOR CAVIUM THUNDERX

Commit e1ac611f ("dt-bindings: PCI: Convert generic host binding to
DT schema") combines all information from pci-thunder-{pem,ecam}.txt
into host-generic-pci.yaml, and deleted the two files in
Documentation/devicetree/bindings/pci/.

Since then, ./scripts/get_maintainer.pl --self-test complains:

  no file matches F: Documentation/devicetree/bindings/pci/pci-thunder-*

As the PCIE DRIVER FOR CAVIUM THUNDERX-relevant information is only a
small part of the host-generic-pci.yaml, do not add this file to the
PCIE DRIVER FOR CAVIUM THUNDERX entry, and only drop the reference to
the removed files.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Rob Herring <robh@kernel.org>