rockchip: board: puma-rk3399: fix warnings in puma_rk3399/fit_spl_atf.its

The ITS file generated warnings due to @<num> designations in the naming
which cause DTC to complain as follows:
  Warning (unit_address_vs_reg): Node /images/uboot@1 has a unit name, but no reg property
  Warning (unit_address_vs_reg): Node /images/atf@1 has a unit name, but no reg property
  Warning (unit_address_vs_reg): Node /images/pmu@1 has a unit name, but no reg property
  Warning (unit_address_vs_reg): Node /images/fdt@1 has a unit name, but no reg property
  Warning (unit_address_vs_reg): Node /configurations/conf@1 has a unit name, but no reg property

This removes the @<num> part from the names, as we only have a single
image for each payload aspect (and only a single configuration) anyway.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>