nand: spl_loaders: Only read enough pages to load the image

All other implementations of nand_spl_load_image only read as many pages as
are necessary to load the image. However, nand_spl_loaders.c loads the full
block. Align it with other load functions so that it is easier to
determine how large of a load buffer we need.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>