riscv/sifive_u: Add QSPI memory region

riscv/sifive_u: Add QSPI memory region

The HiFive Unleashed uses is25wp256 SPI NOR flash. There is currently no
model of this in QEMU, so to allow boot firmware developers to use QEMU
to target the Unleashed let's add a chunk of memory to represent the QSPI0
memory mapped flash. This can be targeted using QEMU's -device loader
command line option.

In the future we can look at adding a model for the is25wp256 flash.

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