rockchip: Don't skip low-level init

At present the low-level init is skipped on rockchip. Among other things
this means that the instruction cache is left disabled. Fix this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-rockchip/rk3288-board-spl.c b/arch/arm/mach-rockchip/rk3288-board-spl.c
index b2c5729..f61eeab 100644
--- a/arch/arm/mach-rockchip/rk3288-board-spl.c
+++ b/arch/arm/mach-rockchip/rk3288-board-spl.c
@@ -270,3 +270,7 @@
 	/* No way to report error here */
 	hang();
 }
+
+void lowlevel_init(void)
+{
+}