fpga: allow programming fpga from FIT image for all FPGA drivers

fpga: allow programming fpga from FIT image for all FPGA drivers

This drops the limit that fpga is only loaded from FIT images for Xilinx.
This is done by moving the 'partial' check from 'common/image.c' to
'drivers/fpga/xilinx.c' (the only driver supporting partial images yet)
and supplies a weak default implementation in 'drivers/fpga/fpga.c'.

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Tested-by: Michal Simek <michal.simek@xilinx.com> (On zcu102)
Signed-off-by: Michal Simek <michal.simek@xilinx.com>