rockchip: boot_mode: add adc dependency for dnl key detection

Most rockchip boards detect the download key via adc,
but there are also some boards don't want to use this
function, they may disable the adc subsystem, so add
the adc dependency here.

Change-Id: I31af5666c8c5e973e9e6b31ce1a3847e0b478840
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>

arch/arm/mach-rockchip/boot_mode.c

@@ -42,6 +42,7 @@ void set_back_to_bootrom_dnl_flag(void)
 
 __weak int rockchip_dnl_key_pressed(void)
 {
+#if defined(CONFIG_ADC)
 	const void *blob = gd->fdt_blob;
 	unsigned int val;
 	int channel = 1;
@@ -65,6 +66,9 @@ __weak int rockchip_dnl_key_pressed(void)
 		return true;
 	else
 		return false;
+#else
+	return false;
+#endif
 }
 
 void devtype_num_envset(void)