configs: engicam: Increase nand kernel partition size

Increase the nand kernel partition size, for supporting
large uImage files, maximum 8MiB.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>