configs: phycore-rk3288: Enable CONFIG_LTO

The phycore-rk3288 SPL binary is reaching the limits of 32KB very often.
Enable CONFIG_LTO to reduce the size of the SPL and make the board more
future proof for changes increasing the SPL size.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>