mtd: Add a CONFIG_SPL_MTD_SUPPORT for a more full NAND subsystem in SPL

This mainly converts the am335x_spl_bch driver to the "normal" format
which means a slight change to nand_info within the driver.

Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Tom Rini <trini@ti.com>