hw/core: Move the ARM sysbus-fdt to core

hw/core: Move the ARM sysbus-fdt to core

The ARM virt machine currently uses sysbus-fdt to create device tree
entries for dynamically created MMIO devices.

The RISC-V virt machine can also benefit from this, so move the code to
the core directory.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-Id: <20220427234146.1130752-3-alistair.francis@opensource.wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>