fpga: region: add register/unregister functions

Another step in separating common code from device tree specific
code for FPGA regions.

* add FPGA region register/unregister functions.
* add the register/unregister functions to the header
* use devm_kzalloc to alloc the region.
* add a method for getting bridges to the region struct
* add priv to the region struct
* use region->info in of_fpga_region_get_bridges

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