fpga: region: check for child regions before allocing image info

During a device tree overlay pre-apply notification, the check
for child FPGA regions can happen slightly earlier.  This saves
us from allocating the FPGA image info that just gets thrown
away.

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