spl/nand: config symbol documentation

Document parameters used for specifying the NAND image to be loaded.

Also fix the definition of CONFIG_SPL_NAND_SIMPLE -- it's only
nand_spl_simple.c, not the entire nand directory.  The word "simple" is
there for a reason.  :-)

Signed-off-by: Scott Wood <scottwood@freescale.com>
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v2: updated for makefile changes earlier in patchset