ARM: rockchip: rock: Correct test to use CONFIG_IS_ENABLED not defined

While it is likely that this entire case is superfluous and can be
removed, correct the test now to match what is in rockchip-common.h and
makes sense based on context of the code.  Otherwise we get a large
number of warnings.

Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Tom Rini <trini@konsulko.com>

include/configs/rock.h

@@ -12,7 +12,7 @@
 
 #define CONFIG_SYS_MMC_ENV_DEV 0
 
-#if defined(ROCKCHIP_BACK_TO_BROM)
+#if CONFIG_IS_ENABLED(ROCKCHIP_BACK_TO_BROM)
 /* SPL @ 32k for 34k
  * u-boot directly after @ 68k for 400k or so
  * ENV @ 992k