rockchip: add timer driver

some rockchip soc will not include lib/timer.c in SPL stage,
so implement timer driver for some soc can use us delay function in SPL.

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 5a4e383..abe03a8 100644
--- a/arch/arm/mach-rockchip/Makefile
+++ b/arch/arm/mach-rockchip/Makefile
@@ -10,4 +10,5 @@
 obj-y += board.o
 endif
 obj-y += common.o
+obj-y += rk_timer.o
 obj-$(CONFIG_ROCKCHIP_RK3288) += rk3288/