lantiq: 4.19: load PCIe driver after PCI driver

lantiq: 4.19: load PCIe driver after PCI driver

In case a PCI device is physical present, the Lantiq PCI driver need to
be loaded prior to the Lantiq PCIe driver. Otherwise none of them will
work because updating the BAR 0 of a device attached to the PCIe Bus
will fail.

Till kernel commit 9667bb039bf6 ("MIPS: lantiq: remove unnecessary
of_platform_default_populate call"), the Lantiq PCI driver was probed
early in the boot process via plat_of_setup().

Since the mentioned commit, both drivers are probed via
of_platform_default_populate_init() and they are probed according to
their order in the devicetree source file.

If the Lantiq PCI driver is enabled in the devicetree, defer the probing
og the Lantiq PCIe driver till the Lantiq PCI driver is probed. Use the
presence of a PCU bus as indicater for a loaded Lantiq PCI driver.

This way we don't need to shuffle the PCI/PCIe nodes in the devicetree
source file, as they are already (correctly) ordered based on the unit
address.

Signed-off-by: Mathias Kresin <dev@kresin.me>