acpi: Support generation of a device

Allow writing an ACPI device 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 build failures on Sandbox]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>