[dev][platform-bus] Remove support for proxying pdev protocol to platform device children

Clients that need this support should now use device_add() instead of pdev_device_add()
and use composite device support to provide platform device protocol to children of
platform devices.

Now that the platform bus no longer needs to support multiple platform devices
in the same devhost, the proxy side of the driver is now greatly simplified.
The functionality that was provided by DeviceProxy has been folded back into PlatformProxy.
DeviceResources now contains only a single set or resources (rather than a tree)
and the proxy protocol no longer needs a device_id.

Disabled gauss PCIE and hikey960 display drivers, which were the only two remaining
uses of this support, and neither of these drivers are currently in use.

TEST: boot and and run full UI on astro. runtests -t platform-bus-test

Change-Id: Ib248f1ce7be3b08c7ddb4c73ad8bf2766c2613f7