riscv: add support for QEMU firmware tables

Enable the QEMU firmware interface if ACPI tables are to be supported on
the QEMU platform.

Enable the QFW MMIO interface if the QEMU firmware interface is enabled.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>