rockchip: Move bootrom helper compilation to a hidden option

Right now the ROCKCHIP_SPL_BACK_TO_BROM option both triggers
compilation of the bootrom hook-code as well as enabling the
behaviour of loading the full U-Boot via the boot.

New added socs may always need the bootrom code, while still
being able to decide between loading U-Boot regularly or via
the bootrom separately.

So move the compilation of the bootrom code to a hidden option
that gets selected by ROCKCHIP_SPL_BACK_TO_BROM, but can also
be selected by other parts.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/mach-rockchip/Kconfig b/arch/arm/mach-rockchip/Kconfig
index 5c4a4c2..a776df3 100644
--- a/arch/arm/mach-rockchip/Kconfig
+++ b/arch/arm/mach-rockchip/Kconfig
@@ -37,11 +37,15 @@
 config ROCKCHIP_SPL_BACK_TO_BROM
 	bool "SPL returns to bootrom"
 	default y if ROCKCHIP_RK3036
+	select ROCKCHIP_BROM_HELPER
 	help
 	  Rockchip SoCs have ability to load SPL & U-Boot binary. If enabled,
           SPL will return to the boot rom, which will then load the U-Boot
           binary to keep going on.
 
+config ROCKCHIP_BROM_HELPER
+	bool
+
 config SPL_MMC_SUPPORT
 	default y if !ROCKCHIP_SPL_BACK_TO_BROM