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/board.c b/arch/arm/mach-rockchip/board.c
index f026abf..380aa91 100644
--- a/arch/arm/mach-rockchip/board.c
+++ b/arch/arm/mach-rockchip/board.c
@@ -45,3 +45,7 @@
 	dcache_enable();
 }
 #endif
+
+void lowlevel_init(void)
+{
+}