dt-bindings: PCI: cadence: Remove "mem" from reg binding

"mem" is not a memory resource and it overlaps with PCIe config space
and memory region. Remove "mem" from reg binding.

Link: https://lore.kernel.org/r/20200722110317.4744-8-kishon@ti.com


Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>