env: ti: k3_dfu: load only the next stage binary

env: ti: k3_dfu: load only the next stage binary

In the TI's K3 bootflow of tiboot3.bin -> tispl.bin -> u-boot.img:
                             (R5 SPL)      (A53 SPL)

We currently provide a common dfu_alt_info_ram for both R5 SPL and A53
SPL which is not intuitive in a regular bootflow where each binary
should only request it's immediate next stage.

This patch updates dfu_alt_info_ram such that the R5 SPL would only
request for tispl.bin and A53 SPL would only request u-boot.img.

Signed-off-by: Anshul Dalal <anshuld@ti.com>