M28: Fix the use of gpmi-nand in mtdparts

The mtd name of the NAND in Linux is "gpmi-nand", not "gpmi-nand.0" as
it would be expected, since the controller doesn't support multiple NANDs
attached to it as of now. Rectify this flub by adjusting default mtdparts.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Stefano Babic <sbabic@denx.de>