dt-bindings: PCI: Convert generic host binding to DT schema

Convert the generic PCI host binding to DT schema. The derivative Juno,
PLDA XpressRICH3-AXI, and Designware ECAM bindings all just vary in
their compatible strings. The simplest way to convert those to
schema is just add them into the common generic PCI host schema.

The HiSilicon ECAM and Cavium ThunderX PEM bindings have an additional
'reg' entry, but are otherwise the same binding as well.

Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Andrew Murray <andrew.murray@arm.com>
Cc: Zhou Wang <wangzhou1@hisilicon.com>
Cc: Will Deacon <will@kernel.org>
Cc: David Daney <david.daney@cavium.com>
Signed-off-by: Rob Herring <robh@kernel.org>