PCI: Move private DT related functions into private header

The functions in linux/of_pci.h are primarily used by host bridge
drivers, so they can be private to drivers/pci/.

The remaining functions are still used mostly in host bridge drivers
that still live in arch specific code. Hopefully someday, those will get
moved into drivers/pci as well.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Jingoo Han <jingoohan1@gmail.com>