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/rv1108/Kconfig b/arch/arm/mach-rockchip/rv1108/Kconfig
index e3a63b8..c5402ab 100644
--- a/arch/arm/mach-rockchip/rv1108/Kconfig
+++ b/arch/arm/mach-rockchip/rv1108/Kconfig
@@ -1,5 +1,8 @@
 if ROCKCHIP_RV1108
 
+choice
+	prompt "RV1108 board select"
+
 config TARGET_EVB_RV1108
 	bool "EVB_RV1108"
 	help
@@ -22,6 +25,8 @@
 	help
 	  RV1108 ELGIN is a board based on the Rockchip RV1108.
 
+endchoice
+
 config SYS_SOC
 	default "rv1108"