hw/riscv: write bootargs 'chosen' FDT after riscv_load_kernel()

hw/riscv: write bootargs 'chosen' FDT after riscv_load_kernel()

The sifive_u, spike and virt machines are writing the 'bootargs' FDT
node during their respective create_fdt().

Given that bootargs is written only when '-append' is used, and this
option is only allowed with the '-kernel' option, which in turn is
already being check before executing riscv_load_kernel(), write
'bootargs' in the same code path as riscv_load_kernel().

Cc: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230102115241.25733-8-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>