hw/riscv: sifive_u: Add a dummy DDR memory controller device

hw/riscv: sifive_u: Add a dummy DDR memory controller device

It is enough to simply map the SiFive FU540 DDR memory controller
into the MMIO space using create_unimplemented_device(), to make
the upstream U-Boot v2020.07 DDR memory initialization codes happy.

Note we do not generate device tree fragment for the DDR memory
controller. Since the controller data in device tree consumes a
very large space (see fu540-hifive-unleashed-a00-ddr.dtsi in the
U-Boot source), and it is only needed by U-Boot SPL but not any
operating system, we choose not to generate the fragment here.
This also means when testing with U-Boot SPL, the device tree has
to come from U-Boot SPL itself, but not the one generated by QEMU
on the fly. The memory has to be set to 8GiB to match the real
HiFive Unleashed board when invoking QEMU (-m 8G).

With this commit, QEMU can boot U-Boot SPL built for SiFive FU540
all the way up to loading U-Boot proper from MMC:

$ qemu-system-riscv64 -nographic -M sifive_u,msel=6 -m 8G -bios u-boot-spl.bin

U-Boot SPL 2020.07-rc3-00208-g88bd5b1 (Jun 08 2020 - 20:16:10 +0800)
Trying to boot from MMC1
Unhandled exception: Load access fault
EPC: 0000000008009be6 TVAL: 0000000010050014

The above exception is expected because QSPI is unsupported yet.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1592268641-7478-6-git-send-email-bmeng.cn@gmail.com
Message-Id: <1592268641-7478-6-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>