83xx: Fix NAND_SPL link address

Apply the same fix for 83xx as was done for 85xx in commit
96196a1f.

Without this, NAND SPLs are built with the text base intended for the main
image, resulting in a broken, very large u-boot-nand.bin.

The block of defines for NAND boot is moved closer to where
CONFIG_SYS_TEXT_BASE is defined.  We can't directly use
CONFIG_SYS_NAND_U_BOOT_DST in the definition of CONFIG_SYS_TEXT_BASE because
autoconf.mk will include the literal text "CONFIG_SYS_NAND_U_BOOT_DST",
but at least keep them close and point out that they're supposed to be
the same.

Signed-off-by: Scott Wood <scottwood@freescale.com>