x86: acpi: Support external GNVS tables

At present U-Boot puts a magic number in the ASL for the GNVS table and
searches for it later.

Add a Kconfig option to use a different approach, where the ASL files
declare the table as an external symbol. U-Boot can then put it wherever
it likes, without any magic numbers or searching.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 256a110..680f26f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -788,6 +788,13 @@
 	  by the operating system. It defines platform-independent interfaces
 	  for configuration and power management monitoring.
 
+config ACPI_GNVS_EXTERNAL
+	bool
+	help
+	  Put the GNVS (Global Non-Volatile Sleeping) table separate from the
+	  DSDT and add a pointer to the table from the DSDT. This allows
+	  U-Boot to better control the address of the GNVS.
+
 endmenu
 
 config HAVE_ACPI_RESUME