commands: Create nand_bitflip command ...

This adds a command to flip bits in a Nand flash. This is useful for
testing purposes to check if flipped bits are corrected and if the
driver returns the correct number of bitflips.

The command writes a configurable number of bitflips to a single Nand
page. If the -r option is not given the results are reproducible, so
calling the same command twice will revert the bitflips.

The command uses the raw read/write Nand operations which are probably
less tested than the regular read/write operations, so the command may
produce surprising results.

As of writing the command has been tested with the GPMI Nand driver and
the imx-nand driver with fixes posted.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>