fpga: bridge: support getting bridge from device

Add two functions for getting the FPGA bridge from the device
rather than device tree node.  This is to enable writing code
that will support using FPGA bridges without device tree.
Rename one old function to make it clear that it is device
tree-ish.  This leaves us with 3 functions for getting a bridge:

* fpga_bridge_get
  Get the bridge given the device.

* fpga_bridges_get_to_list
  Given the device, get the bridge and add it to a list.

* of_fpga_bridges_get_to_list
  Renamed from priviously existing fpga_bridges_get_to_list.
  Given the device node, get the bridge and add it to a list.

Signed-off-by: Alan Tull <atull@kernel.org>
Acked-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>