cmd, nand: add an option to disable the verification when writing in raw mode

Modern NANDs do not guarantee that data written in raw mode will not
contain bitflips just after writing them. This is fine since the number
of bitflips should be rather low and thus fixable by the ECC engine,
but since we are reading data in raw mode to verify if they match the
input data we cannot prevent failures if some bits are flipped.

The option of using standard mode to verify the data is not acceptable
either, since one of the usage of raw mode is to allow flashing images
that do not respect the standard NAND page layout or the default ECC
config (this is the case on Allwinner platforms, where the ROM code
tests several hardcoded configs, which are not necessarily matching the
NAND characteristics).

Add an extension to the nand write.raw command allowing one to disable
the verification step.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>