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>

include/fpga.h

@@ -54,6 +54,7 @@ void fpga_init(void);
 int fpga_add(fpga_type devtype, void *desc);
 int fpga_count(void);
 const fpga_desc *const fpga_get_desc(int devnum);
+int fpga_is_partial_data(int devnum, size_t img_len);
 int fpga_load(int devnum, const void *buf, size_t bsize,
 	      bitstream_type bstype);
 int fpga_fsload(int devnum, const void *buf, size_t size,