pci: Support parsing PCI controller DT subnodes

The PCI controller can have DT subnodes describing extra properties
of particular PCI devices, ie. a PHY attached to an EHCI controller
on a PCI bus. This patch parses those DT subnodes and assigns a node
to the PCI device instance, so that the driver can extract details
from that node and ie. configure the PHY using the PHY subsystem.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>