spl: fit: nand: allow for non-page-aligned elements

Add a weak nand_get_mtd function for nand drivers to provide mtd info
and use this to set pagesize such that reading of non page-aligned
elements can succeed.

The spl_load_simple_fit already handles block block access so all we
need to do is provide the nand writesize as the block length.

Further cleanup of the drivers which use nand_spl_loaders.c such as
am335x_spl_bch.c, atmel_nand.c, and nand_spl_simple.c could be done
using info from mtd_info instead of statically defined details.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Tom Rini <trini@konsulko.com>