fpga: region: separate out code that parses the overlay

New function of_fpga_region_parse_ov added, moving code
from fpga_region_notify_pre_apply.  This function
gets the FPGA image info from the overlay and is able
to simplify some of the logic involved.

This is a baby step in refactoring the FPGA region code to
separate out common code from Device Tree overlay 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>