acpi: Add a method to write tables for a device

A device may want to write out ACPI tables to describe itself to Linux.
Add a method to permit this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index df9f183..4bccfbe 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -226,6 +226,10 @@
 		compatible = "denx,u-boot-acpi-test";
 	};
 
+	acpi-test2 {
+		compatible = "denx,u-boot-acpi-test";
+	};
+
 	clocks {
 		clk_fixed: clk-fixed {
 			compatible = "fixed-clock";