acpi: Support writing named values

Allow writing named integers and strings to the generated ACPI code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
[bmeng: Fix the "new blank line at EOF" warning]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>