fpga: clarify that unregister functions also free

The following functions also free the struct.  Add that
fact to the function documentation.
 - fpga_mgr_free
 - fpga_bridge_free
 - fpga_region_free

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