acpi: Move acpi_add_table() to generic code

Move this code to a generic location so that we can test it with sandbox.
This requires adding a few new fields to acpi_ctx, so drop the local
variables used in the original code.

Also use mapmem to avoid pointer-to-address casts which don't work on
sandbox.

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