cmd: fpga: Move fpga_loadbitstream to fpga.c

In bitstream decoding you can directly check device
which you want to load and in fpga.c are fpga_validate
and fpga_dev_info functions which should be used for it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@ti.com>