ppc: Move wdt_last to arch_global_data

Move this field into arch_global_data and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/lwmon5/lwmon5.c b/board/lwmon5/lwmon5.c
index ecd9536..29e24fb 100644
--- a/board/lwmon5/lwmon5.c
+++ b/board/lwmon5/lwmon5.c
@@ -357,16 +357,16 @@
 	 * Don't allow watch-dog triggering more frequently than
 	 * the predefined value CONFIG_WD_MAX_RATE [ticks].
 	 */
-	if (ct >= gd->wdt_last) {
-		if ((ct - gd->wdt_last) < CONFIG_WD_MAX_RATE)
+	if (ct >= gd->arch.wdt_last) {
+		if ((ct - gd->arch.wdt_last) < CONFIG_WD_MAX_RATE)
 			return;
 	} else {
 		/* Time base counter had been reset */
-		if (((unsigned long long)(-1) - gd->wdt_last + ct) <
+		if (((unsigned long long)(-1) - gd->arch.wdt_last + ct) <
 		    CONFIG_WD_MAX_RATE)
 			return;
 	}
-	gd->wdt_last = get_ticks();
+	gd->arch.wdt_last = get_ticks();
 #endif
 
 	/*