nand_spl: Support page-aligned read in nand_load, use chipselect

Supporting page-aligned reads doesn't incure any sinificant overhead, just
a small change in the algorithm. Also replace in_8 with readb, since there
is no in_8 on ARM.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Signed-off-by: Scott Wood <scottwood@freescale.com>