Fix regression in SMDK6400

Fix regression in SMDK6400

s3c64xx.c implemented its own nand_read_byte, nand_write_buf and
nand_read_buf functions. This provoked a regression when these functions
were made public by patch 55f429bb39614a16b1bacc9a8bea9ac01a60bfc8.

This deletes these duplicated functions from s3c64xx.c and adds the generic
implementations in nand_base.c to the spl Makefile. It also adds
-ffcuntion-sections and -gc-sections to the compilation flags of the SPL to
avoid errors originating from unused functions in nand_base.c.

Description of the regression:
http://article.gmane.org/gmane.comp.boot-loaders.u-boot/108873

Signed-off-by: Simon Schwarz <simonschwarzcor@gmail.com>
Cc: scottwood@freescale.com
Cc: s-paulraj@ti.com
Cc: albert.u.boot@aribaud.net