common: Move reset_cpu() to the CPU header

Move this function out of common.h and into a relevant header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/watchdog/imx_watchdog.c b/drivers/watchdog/imx_watchdog.c
index c030360..62bf25e 100644
--- a/drivers/watchdog/imx_watchdog.c
+++ b/drivers/watchdog/imx_watchdog.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <cpu_func.h>
 #include <dm.h>
 #include <asm/io.h>
 #include <wdt.h>