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/lib/acpi/Makefile b/lib/acpi/Makefile
index 4674a92..9f70fe6 100644
--- a/lib/acpi/Makefile
+++ b/lib/acpi/Makefile
@@ -10,4 +10,5 @@
 # With QEMU the ACPI tables come from there, not from U-Boot
 ifndef CONFIG_QEMU
 obj-y += base.o
+obj-y += facs.o
 endif