SPL: ONENAND: Fix onenand_spl_load_image implementation.

Tested with an IGEPv2 board seems that current onenand_spl_load_image implementation
doesn't work. This patch fixes this function changing the read loop and reading the
onenand blocks from page to page.

Tested with various IGEP based boards with a OneNAND from Numonyx.

Signed-off-by: Enric Balletbo i Serra <eballetbo@iseebcn.com>