fpga: region: add fpga_region_class_find

Add a function for searching the fpga-region class.  This
will be useful when device tree code is no longer in the
same file that declares the fpga-region class.  Another
step in separating common FPGA region code from device
tree support.

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