PCI: rcar: Add R-Car gen3 PHY support

On R-Car gen3 SoCs the PCIe PHY has its own register region, thus we
need to add the corresponding code in rcar_pcie_hw_init_gen3() and call
devm_phy_optional_get() at the driver's probing time, so that the
existing R-Car gen3 device trees (not having a PHY node) would still
work (we only need to power up the PHY on R-Car V3H).

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Rob Herring <robh@kernel.org>