tools: kwboot: Update doc about Avanta

Testes proved that current kwboot version supports also Avanta SoCs.
It looks like that Avanta SoCs are using same kwbimage format as Armada.

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>