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/board/technexion/pico-imx6ul/spl.c b/board/technexion/pico-imx6ul/spl.c
index 7f520be..ae51113 100644
--- a/board/technexion/pico-imx6ul/spl.c
+++ b/board/technexion/pico-imx6ul/spl.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0+
 
+#include <cpu_func.h>
 #include <asm/arch/clock.h>
 #include <asm/arch/iomux.h>
 #include <asm/arch/imx-regs.h>