rockchip: rk3188: migrate to use common spl board file

rk3188 has similar boot flow in SPL with other Rockchip SoCs,
migrate to use common spl board file.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/mach-rockchip/Kconfig b/arch/arm/mach-rockchip/Kconfig
index 2b4e3c7..66d07f6 100644
--- a/arch/arm/mach-rockchip/Kconfig
+++ b/arch/arm/mach-rockchip/Kconfig
@@ -36,6 +36,7 @@
 	select SPL_ROCKCHIP_EARLYRETURN_TO_BROM
 	select SPL_ROCKCHIP_BACK_TO_BROM
 	select BOARD_LATE_INIT
+	imply SPL_ROCKCHIP_COMMON_BOARD
 	help
 	  The Rockchip RK3188 is a ARM-based SoC with a quad-core Cortex-A9
 	  including NEON and GPU, 512KB L2 cache, Mali-400 graphics, two