mtd: mxc_nand: fix truncate of unaligned oob copying
Copy to/from oob io area might not be aligned to 4 bytes. When 8 bit ECC is
used, the buffer size is 26. Add memcpy16_{to,from}io, and use them to avoid
truncating the buffer. Prefer memcpy32_{to,from}io when the buffer is properly
aligned for better performance.