mtd: mtd_nandecctest: add double bit error detection tests

This adds the double bit error detection test cases listed below:

* Prepare data block with double bit error and ECC data without
corruption, and verify that the uncorrectable error is detected by
__nand_correct_data().

* Prepare data block with single bit error and ECC data with single bit
error, and verify that the uncorrectable error is detected.

* Prepare data block without corruption and ECC data with double bit
error, and verify that the uncorrectable error is detected.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>