imx-bbu-nand-fcb: Use barebox partition instead of whole device
We used to use nand0 device for storing barebox and made the assumption
that there is enough space at the beginning of the first device. Instead,
use the barebox partition directly. This requires that the partition
where barebox should be stored is named 'barebox', that is the case for
all boards currently.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>