x86: Move FACS table to a writer function

Move this table over to use a writer function, moving the code from the
x86 implementation.

Add a pointer to the DSDT in struct acpi_ctx so we can reference it later.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/acpi.h b/include/dm/acpi.h
index 2021a69..c4baeb6 100644
--- a/include/dm/acpi.h
+++ b/include/dm/acpi.h
@@ -52,6 +52,7 @@
  *	adding a new table. The RSDP holds pointers to the RSDT and XSDT.
  * @rsdt: Pointer to the Root System Description Table
  * @xsdt: Pointer to the Extended System Description Table
+ * @facs: Pointer to the Firmware ACPI Control Structure
  * @nhlt: Intel Non-High-Definition-Audio Link Table (NHLT) pointer, used to
  *	build up information that audio codecs need to provide in the NHLT ACPI
  *	table
@@ -65,6 +66,7 @@
 	struct acpi_rsdp *rsdp;
 	struct acpi_rsdt *rsdt;
 	struct acpi_xsdt *xsdt;
+	struct acpi_facs *facs;
 	struct nhlt *nhlt;
 	char *len_stack[ACPIGEN_LENSTACK_SIZE];
 	int ltop;