mtd: mtd_nandecctest: rewrite the test routine

This rewrites the entire test routine in order to make it easy to add more
tests by later changes and minimize duplication of each tests as much as
possible.

Now that each test is described by the members of struct nand_ecc_test:
- name: descriptive testname
- prepare: function to prepare data block and ecc with artifical corruption
- verify: function to verify the result of correcting data block

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>