imx: Add 2GB lpddr support for i.MX8MN Beacon EmbeddedWorks devkit.

There is a second lpddr configuration with 2GB of RAM, but this requires
different RAM timings, so in addition to adding the timing file, a
separate defconfig is necessary.

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/board/beacon/imx8mn/Kconfig b/board/beacon/imx8mn/Kconfig
index 6ed17c0..2bcfb25 100644
--- a/board/beacon/imx8mn/Kconfig
+++ b/board/beacon/imx8mn/Kconfig
@@ -13,6 +13,10 @@
 	bool "Force to use nominal mode for SOC and ARM"
 	default n
 
+config IMX8MN_BEACON_2GB_LPDDR
+	bool "Enable 2GB LPDDR"
+	default n
+
 source "board/freescale/common/Kconfig"
 
 endif