ARM: enable LTO for some boards

Enable LTO for some boards that were tested by people on U-Boot Mailing
List.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Tested-by: Adam Ford <aford173@gmail.com>
Tested-by: Pali Rohár <pali@kernel.org>
Tested-by: Tim Harvey <tharvey@gateworks.com>