omap-common: reorganize spl.c

split-up spl.c into spl.c, spl_mmc.c and spl_nand.c. This avoids problems
with missing defines if a board does not use mmc or nand. This includes
adding spl_ prefix to some functions which are now public. spl_image_t is now
a public type. Added some of the common functions to omap-common.h

Signed-off-by: Simon Schwarz <simonschwarzcor@gmail.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>