dt-bindings: PCI: rcar-pci-host: Convert bindings to json-schema

Convert Renesas PCIe Host controller bindings documentation to
json-schema. Note that some compatible doesn't contain on
the original documentation so that incremental patches are required
for it.

Link: https://lore.kernel.org/r/1604455096-13923-2-git-send-email-yoshihiro.shimoda.uh@renesas.com
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>