acpi: Support writing a GPIO

Allowing writing out a reference to a GPIO within the ACPI output. This
can be used by ACPI code to access a GPIO at runtime.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
3 files changed