configs: engicam: Rename nand with gpmi-name in mtdparts

gpmi-nand is the proper name used in nand driver from Linux for all
imx related nand boards, so rename mtdparts name as gpmi-nand instead
of nand, this will eventually reflects all nand info to Linux from
u-boot like mtdparts.

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>