imx8m: Enable LTO by default

imx8m: Enable LTO by default

In an attempt to select ARMV8_SPL_EXCEPTION_VECTORS, the SPL size
could not fit into the internal SRAM of some imx8m targets:

   aarch64:  +   imx8mm_phg
+aarch64-linux-ld.bfd: u-boot-spl section `__u_boot_list' will not fit in region `.sram'
+aarch64-linux-ld.bfd: region `.sram' overflowed by 1824 bytes

Select LTO to prevent that.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>