Correct SPL use of IMX8MN_BEACON_2GB_LPDDR

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_IMX8MN_BEACON_2GB_LPDDR defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Adam Ford <aford173@gmail.com>
diff --git a/include/configs/imx8mn_beacon.h b/include/configs/imx8mn_beacon.h
index bb3dfe3..1880d03 100644
--- a/include/configs/imx8mn_beacon.h
+++ b/include/configs/imx8mn_beacon.h
@@ -80,7 +80,7 @@
 
 #define CFG_SYS_SDRAM_BASE		0x40000000
 #define PHYS_SDRAM			0x40000000
-#if CONFIG_IS_ENABLED(IMX8MN_BEACON_2GB_LPDDR)
+#if IS_ENABLED(CONFIG_IMX8MN_BEACON_2GB_LPDDR)
 #define PHYS_SDRAM_SIZE		0x80000000 /* 2GB DDR */
 #else
 #define PHYS_SDRAM_SIZE		0x40000000 /* 1GB DDR */