configs: at91: sama5d2_icp_mmc: Enable CONFIG_LTO

Adding just a tiny bit more code for sama5d2_icp_mmc leads to a SRAM
image overflow. Fix this by enabling LTO for this board, so that such
changes still can be made to the common U-Boot code.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Pali Rohár <pali@kernel.org>
Tested-by: Mihai Sain <mihai.sain@microchip.com>
[eugen.hristev@microchip.com: restrict patch just to CONFIG_LTO]
Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>