fpga: add attribute groups

Make it easy to add attributes to low level FPGA drivers the right
way.  Add attribute groups pointers to structures that are used when
registering a manager, bridge, or group.  When the low level driver
registers, set the device attribute group.  The attributes are
created in device_add.

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