rockchip: fixup board choice in Kconfig

Kconfig for board target select is choice option, fixup for
rk3036, rk3288 and rv1108.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/mach-rockchip/rk3036/Kconfig b/arch/arm/mach-rockchip/rk3036/Kconfig
index 5e04d20..0f6b5c4 100644
--- a/arch/arm/mach-rockchip/rk3036/Kconfig
+++ b/arch/arm/mach-rockchip/rk3036/Kconfig
@@ -1,5 +1,8 @@
 if ROCKCHIP_RK3036
 
+choice
+	prompt "RK3036 board select"
+
 config TARGET_EVB_RK3036
 	bool "EVB_RK3036"
 	select BOARD_LATE_INIT
@@ -8,6 +11,8 @@
 	bool "KYLIN_RK3036"
 	select BOARD_LATE_INIT
 
+endchoice
+
 config SYS_SOC
 	default "rk3036"