mtd: otp: Put factory OTP/NVRAM into the entropy pool

The factory OTP, if supported, contains factory-programmed
information such as typically the serial number or production
week for the chip.

As this is device-unique information, submit it into the
system entropy pool.

This does not count as improvement of the entropy as such
but in practice it makes it a bit more random to mix in these
numbers.

Cc: Michael Walle <michael@walle.cc>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230606142931.3721374-1-linus.walleij@linaro.org