ARM: imx: Enable LTO for DH electronics i.MX6 DHCOM

Enable LTO to reduce the size of SPL, which with SPL SDP
support may be close to the limit.

Signed-off-by: Marek Vasut <marex@denx.de>
diff --git a/configs/dh_imx6_defconfig b/configs/dh_imx6_defconfig
index 29b0782..fa2d7f3 100644
--- a/configs/dh_imx6_defconfig
+++ b/configs/dh_imx6_defconfig
@@ -28,6 +28,7 @@
 CONFIG_AHCI=y
 CONFIG_SYS_MEMTEST_START=0x10000000
 CONFIG_SYS_MEMTEST_END=0x20000000
+CONFIG_LTO=y
 CONFIG_SYS_MONITOR_LEN=409600
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y