MAINTAINERS: Add arch/x86/pci to PCI file patterns

I handle many arch/x86/pci changes, so help those patches get to the
PCI patchwork.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Acked-by: Ingo Molnar <mingo@kernel.org>