hw/riscv/sifive_u: Create a SiFive U SoC object

Create a SiFive Unleashed U54 SoC and use that in the sifive_u machine.

We leave the SoC, RAM, device tree and reset/fdt loading as part of the
machine. All the other device creation has been moved to the SoC.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Michael Clark <mjc@sifive.com>