hw/riscv/boot.c: use MachineState in riscv_load_initrd()

hw/riscv/boot.c: use MachineState in riscv_load_initrd()

'filename', 'mem_size' and 'fdt' from riscv_load_initrd() can all be
retrieved by the MachineState object for all callers.

Cc: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230102115241.25733-9-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>