PCI: rockchip: Add remove() support

Currently, if we try to unbind the platform device, the remove will
succeed, but the removal won't undo most of the registration, leaving
partially-configured PCI devices in the system.

This allows, for example, a simple 'lspci' to crash the system, as it will
try to touch the freed (via devm_*) driver structures, e.g., on RK3399:

  # echo f8000000.pcie > /sys/bus/platform/drivers/rockchip-pcie/unbind
  # lspci

So let's implement device remove().

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Shawn Lin <shawn.lin@rock-chips.com>