acpi: Support generation of ACPI code

acpi: Support generation of ACPI code

Add a new file to handle generating ACPI code programatically. This is
used when information must be dynamically added to the tables, e.g. the
SSDT.

Initial support is just for writing simple values. Also add a 'base' value
so that the table can be freed. This likely doesn't happen in normal code,
but is nice to do in tests.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>