fpga: add fpga bridge framework

This framework adds API functions for enabling/
disabling FPGA bridges under kernel control.

This allows the Linux kernel to disable FPGA bridges
during FPGA reprogramming and to enable FPGA bridges
when FPGA reprogramming is done.  This framework is
be manufacturer-agnostic, allowing it to be used in
interfaces that use the FPGA Manager Framework to
reprogram FPGA's.

The functions are:
* of_fpga_bridge_get
* fpga_bridge_put
   Get/put an exclusive reference to a FPGA bridge.

* fpga_bridge_enable
* fpga_bridge_disable
   Enable/Disable traffic through a bridge.

* fpga_bridge_register
* fpga_bridge_unregister
   Register/unregister a device-specific low level FPGA
   Bridge driver.

Get an exclusive reference to a bridge and add it to a list:
* fpga_bridge_get_to_list

To enable/disable/put a set of bridges that are on a list:
* fpga_bridges_enable
* fpga_bridges_disable
* fpga_bridges_put

Signed-off-by: Alan Tull <atull@opensource.altera.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>