arm: enable support for QEMU firmware tables

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

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