hw/riscv/sifive_u.c: simplify create_fdt()

hw/riscv/sifive_u.c: simplify create_fdt()

'cmdline' isn't being used. Remove it.

A MachineState pointer is being retrieved via a MACHINE() macro calling
qdev_get_machine(). Use MACHINE(s) instead to avoid calling qdev().

 'mem_size' is being set as machine->ram_size by the caller. Retrieve it
via ms->ram_size.

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: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230111170948.316276-4-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>