acpi: Kconfig symbol CONFIG_QFW_ACPI

We have two implementations of write_acpi_tables(). One for writing ACPI
tables based on ACPI_WRITER() entries another based on copying tables from
QEMU.

Create a symbol CONFIG_QFW_ACPI that signifies copying ACPI tables from
QEMU and use it consistently.

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