ARMv8/sec_firmware: Convert to use fit_get_data_conf_prop

This reduces sec_firmware_get_data to a single call to
fit_get_data_conf_prop. I think sec_firmware_check_copy_loadable could also
be converted, but it does not map as straightforwardly, so I have left it
for a future cleanup.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>