configs: j721e_evm_a72_defconfig: Fix the bootcmd

Add the command "boot_rprocs" that is required for booting remote
processors in U-Boot.

Fixes: 5980925e ("include: configs: j721e_evm: Add support to boot ethfw core in j721e")
Reported-by: Jesse Villarreal <jesse.villarreal@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>