rockchip: add config decide whether to build common.c

some rockchips soc will not use uclass in SPL stage,
so define config to decide whether to build common.c

Signed-off-by: Lin Huang <hl@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-rockchip/Makefile b/arch/arm/mach-rockchip/Makefile
index 7088711..902235b 100644
--- a/arch/arm/mach-rockchip/Makefile
+++ b/arch/arm/mach-rockchip/Makefile
@@ -9,6 +9,6 @@
 else
 obj-y += board.o
 endif
-obj-y += common.o
 obj-y += rk_timer.o
+obj-$(CONFIG_$(SPL_)ROCKCHIP_COMMON) += common.o
 obj-$(CONFIG_ROCKCHIP_RK3288) += rk3288/