acpi: extend aml_add() to accept target argument

acpi: extend aml_add() to accept target argument

it allows to express following ASL expression:
 Add(arg1, arg2, result)

usecases that do not need to store result
should pass NULL as 3rd arg that would express
 Add(arg1, arg2,)
construct.

Signed-off-by: Igor Mammedov <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Marcel Apfelbaum <[email protected]>