configs: sama5d2: enable CONFIG_LTO

arm-none-linux-gnueabihf-ld.bfd: u-boot-spl section `__u_boot_list' will not fit in region `.sram'
arm-none-linux-gnueabihf-ld.bfd: region `.sram' overflowed by 100 bytes

SPL is at limit so to stop seeing above error in built, enable
link time optimizations CONFIG_LTO.

Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Tested-by: Mihai Sain <mihai.sain@microchip.com>