PCI/pwrctl: Create platform devices for child OF nodes of the port node

In preparation for introducing PCI device power control - a set of
library functions that will allow powering-up of PCI devices before
they're detected on the PCI bus - we need to populate the devices
defined on the device-tree.

We are reusing the platform bus as it provides us with all the
infrastructure we need to match the pwrctl drivers against the
compatibles from OF nodes.

These platform devices will be probed by the driver core and bound to
the PCI pwrctl drivers we'll introduce later.

Tested-by: Amit Pundir <amit.pundir@linaro.org>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550-QRD, SM8650-QRD & SM8650-HDK
Tested-by: Caleb Connolly <caleb.connolly@linaro.org> # OnePlus 8T
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20240612082019.19161-4-brgl@bgdev.pl


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>