dt-bindings: PCI: intel,lgm-pcie: Fix matching on all snps,dw-pcie instances

The intel,lgm-pcie binding is matching on all snps,dw-pcie instances
which is wrong. Add a custom 'select' entry to fix this.

Fixes: e54ea45a ("dt-bindings: PCI: intel: Add YAML schemas for the PCIe RC controller")
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: linux-pci@vger.kernel.org
Reviewed-by: Dilip Kota <eswara.kota@linux.intel.com>
Signed-off-by: Rob Herring <robh@kernel.org>