Add a custom constant for the ".bus_shift" initialiser to capture a
non-standard platform-specific ECAM bus shift value.

Standard values otherwise defined in the PCI Express Specification are
available in the include/linux/pci-ecam.h.
Suggested-by: NBjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20201129230743.3006978-3-kw@linux.comSigned-off-by: NKrzysztof Wilczyński <kw@linux.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>