board: theobroma-systems: Update TF-A setup steps for RK3588 boards

board: theobroma-systems: Update TF-A setup steps for RK3588 boards

ddrbin_tool interface has been changed. Additional chip_name argument
is now required to modify ddr binary file. Update documentation
to be consistent with the new interface.

Update BL31 and ROCKCHIP_TPL file paths to match current version
of binaries available in the rkbin repository.

Signed-off-by: Daniel Semkowicz <dse@thaumatec.com>
Acked-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>