hw/riscv/spike.c: load initrd right after riscv_load_kernel()

hw/riscv/spike.c: load initrd right after riscv_load_kernel()

This will make the code more in line with what the other boards are
doing. We'll also avoid an extra check to machine->kernel_filename since
we already checked that before executing riscv_load_kernel().

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Message-Id: <20230102115241.25733-6-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>