MTD: nand: mxs_nand: Allow driver to auto setup ECC in SPL

MTD: nand: mxs_nand: Allow driver to auto setup ECC in SPL

The initialization of the NAND in SPL hard-coded ecc.bytes,
ecc.size, and ecc.strength which may work for some NAND parts,
but it not appropriate for others.  With the pending patch
"mxs_nand: Fix BCH read timeout error on boards requiring ECC"
the driver can auto configure the ECC when these entries are
blank.  This patch has been tested in NAND flash with oob 64
and oob 128.

Signed-off-by: Adam Ford <aford173@gmail.com>
Tested-by: Jörg Krause <joerg.krause@embedded.rocks>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Tested-by: Tim Harvey <tharvey@gateworks.com>