hw/riscv: sifive_u: Add QSPI2 controller and connect an SD card

hw/riscv: sifive_u: Add QSPI2 controller and connect an SD card

This adds the QSPI2 controller to the SoC, and connects an SD
card to it. The generation of corresponding device tree source
fragment is also added.

Specify machine property `msel` to 11 to boot the same upstream
U-Boot SPL and payload image for the SiFive HiFive Unleashed board.
Note subsequent payload is stored in the SD card image.

$ qemu-system-riscv64 -nographic -M sifive_u,msel=11 -smp 5 -m 8G \
    -bios u-boot-spl.bin -drive file=sdcard.img,if=sd

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210126060007.12904-6-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>