observed issue.  rsdt pointed to incomplete fadt, leading to bogus
fixed event blocks.

solution.  prefer revision 2 of the table.

Reference: /n/atom/patch/applied2013/acpiprevrev2
Date: Mon Oct  7 06:28:25 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/acpi.c	Mon Oct  7 06:27:18 2013
+++ /sys/src/nix/k10/acpi.c	Mon Oct  7 06:27:18 2013
@@ -192,14 +192,13 @@
 static void
 maptables(void)
 {
-	loadrsd();
 	if(ntblmap > 0 || ntblpa > 0)
 		return;
-	if(!checksum(rsd, 20))
+	loadrsd();
+	if(rsd->rev >= 2 && !checksum(rsd, 36))
+		maptable(get64(rsd->xaddr));
+	else if(!checksum(rsd, 20))
 		maptable(get32(rsd->raddr));
-	if(rsd->rev >= 2)
-		if(!checksum(rsd, 36))
-			maptable(get64(rsd->xaddr));
 }
 
 enum {