MAINTAINERS: Move to shared PCI tree

Move PCI subsystem maintenance to a shared git tree to make it easier for
maintainers to collaborate.  Update MAINTAINERS accordingly.  No change to
patch submission and patchwork tracking.

Link: https://lore.kernel.org/r/20230126211003.1310916-1-helgaas@kernel.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
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