arm64: imx: imx8mp-beacon: Enable LTO

With LTO enabled, SPL shrinks about 10K and U-Boot shrinks
about 30K.

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/imx8mp_beacon_defconfig b/configs/imx8mp_beacon_defconfig
index 99c4043..77a10b9 100644
--- a/configs/imx8mp_beacon_defconfig
+++ b/configs/imx8mp_beacon_defconfig
@@ -18,6 +18,7 @@
 CONFIG_TARGET_IMX8MP_BEACON=y
 CONFIG_SYS_PROMPT="u-boot=> "
 CONFIG_SYS_MONITOR_LEN=524288
+CONFIG_LTO=y
 CONFIG_SPL_MMC=y
 CONFIG_SPL_SERIAL=y
 CONFIG_SPL_DRIVERS_MISC=y