Documentation: arm: Add bootargs to the table of added DT parameters

The bootargs node is also added by the EFI stub in the function
update_fdt(), so add it to the table.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Song Shuai <songshuaishuai@tinylab.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230817130734.10387-1-alexghiti@rivosinc.com