cmd: fpga: Convert to use fit_get_data_node

This converts the FIT loading process of the fpga command to use
fit_get_data_node.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>