mtd: nand: mxs_nand_spl: Remove the page aligned access

The mxs_nand_spl driver can support to read from page unaligned offset,
so don't need to set bl_len to ask spl_load_simple_fit to handle
the page unaligned access.

Actually spl_load_simple_fit has two parts of reading:
spl_simple_fit_read and spl_load_fit_image.
The spl_load_fit_image can handle the page unaligned offset,
but the spl_simple_fit_read can't do it. spl_simple_fit_read requires
the FIT location at page aligned offset.

Hence, remove the nand_get_mtd overwrite function from mxs_nand_spl
to use page unaligned read by driver.

Signed-off-by: Ye Li <ye.li@nxp.com>
Tested-by: Tim Harvey <tharvey@gateworks.com> #gw_ventana