x86: Move DSDT table to a writer function

x86: Move DSDT table to a writer function

Move this table over to use a writer function, moving the code from the
x86 implementation.

Add a pointer to the DSDT in struct acpi_ctx so we can reference it later.

Disable this table for sandbox since we don't actually compile real ASL
code.

Signed-off-by: Simon Glass <sjg@chromium.org>