ARM: renesas: Enable LTO on R-Car

Enable LTO globally on Renesas R-Car platforms. This has been enabled
on a subset of boards already, but at this point it is safe to enable
it globally. This saves units or tens of kiB from the resulting build.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>