[U-Boot] smdkv310: fix needed for out-of-tree build

Dear Chander,

when building the "smdkv310" board out-of-tree, it pollutes the source directory with the following files:

mmc_spl/board/samsung/smdkv310/lowlevel_init.S mmc_spl/board/samsung/smdkv310/mem_setup.S mmc_spl/board/samsung/smdkv310/start.S

which then never get cleaned up.

Please fix your Makefile for out-of-tree building.

Thanks.

Best regards,

Wolfgang Denk