docs/system/riscv: sifive_u: Document '-dtb' usage

Update the 'sifive_u' machine documentation to mention the '-dtb'
option that can be used to pass a custom DTB to QEMU.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210430071302.1489082-6-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>