hw/arm/virt-acpi-build.c: Migrate fw_cfg creation to common location

hw/arm/virt-acpi-build.c: Migrate fw_cfg creation to common location

RISC-V also needs to use the same code to create fw_cfg in DSDT. So,
avoid code duplication by moving the code in arm and riscv to a device
specific file.

Suggested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20231218150247.466427-2-sunilvl@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>