Per PCIe 5.0 r1.0, Terms and Acronyms, Page 80:

  Reserved register fields must be read only and must return 0 (all 0's
  for multi-bit fields) when read. Reserved encodings for register and
  packet fields must not be used. Any implementation dependence on a
  Reserved field value or encoding will result in an implementation that
  is not PCI Express-compliant.

This patch ensures reads will return 0 for any bit not in the Read-Only,
Read-Write, or Write-1-to-Clear bitmasks.

Link: https://lore.kernel.org/r/20200511162117.6674-5-jonathan.derrick@intel.comSigned-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>