rockchip: add description for TPL_ROCKCHIP_COMMON_BOARD

SPL_ROCKCHIP_COMMON_BOARD, an almost identical option, has a title but
this one doesn't for some reason. Add a description to make the menu
easier to read.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/mach-rockchip/Kconfig b/arch/arm/mach-rockchip/Kconfig
index d8d68ba..b689a42 100644
--- a/arch/arm/mach-rockchip/Kconfig
+++ b/arch/arm/mach-rockchip/Kconfig
@@ -306,7 +306,7 @@
 	  no TPL for the board.
 
 config TPL_ROCKCHIP_COMMON_BOARD
-	bool ""
+	bool "Rockchip TPL common board file"
 	depends on TPL
 	help
 	  Rockchip SoCs have similar boot process, prefer to use TPL for DRAM