riscv/sifive_u: Add the start-in-flash property

riscv/sifive_u: Add the start-in-flash property

Add a property that when set to true QEMU will jump from the ROM code to
the start of flash memory instead of DRAM which is the default
behaviour.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>