acpi: Create a new Kconfig for ACPI

We have several Kconfig options for ACPI, but all relate to specific
functions, such as generating tables and AML code.

Add a new option which controls including basic ACPI library code,
including the lib/acpi directory. This will allow us to add functions
which are available even if table generation is not supported.

Adjust the command to avoid a build error when ACPIGEN is not enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/lib/Kconfig b/lib/Kconfig
index d8dac09..c8b3ec1 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -281,9 +281,17 @@
 	  U-Boot can generate these tables and pass them to the Operating
 	  System.
 
+config ACPI
+	bool "Enable support for ACPI libraries"
+	depends on SUPPORT_ACPI
+	help
+	  Provides library functions for dealing with ACPI tables. This does
+	  not necessarily include generation of tables
+	  (see GENERATE_ACPI_TABLE), but allows for tables to be located.
+
 config GENERATE_ACPI_TABLE
 	bool "Generate an ACPI (Advanced Configuration and Power Interface) table"
-	depends on SUPPORT_ACPI
+	depends on ACPI
 	select QFW if QEMU
 	help
 	  The Advanced Configuration and Power Interface (ACPI) specification