acpi: Support writing Device Properties objects via _DSD

More complex device properties can be provided to drivers via a
device-specific data (_DSD) object.

To create this we need to build it up in a separate data structure and
then generate the ACPI code, due to its recursive nature.

Add an implementation of this.

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>
diff --git a/include/acpi/acpigen.h b/include/acpi/acpigen.h
index 31fa20a..b10226f 100644
--- a/include/acpi/acpigen.h
+++ b/include/acpi/acpigen.h
@@ -31,6 +31,7 @@
 	PACKAGE_OP		= 0x12,
 	DUAL_NAME_PREFIX	= 0x2e,
 	MULTI_NAME_PREFIX	= 0x2f,
+	ROOT_PREFIX		= 0x5c,
 };
 
 /**