rockchip: puma-rk3399: migrate to u-boot-rockchip-spi.bin

Now that a single binary containing TPL/SPL correctly formatted for SPI
flashes and U-Boot proper, can be generated by binman, let's do it.

Also update the documentation to tell the user to use this newly
generated file instead of manually generating and flashing the binaries.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>