ARM: dts: imx28-m28: Remove unneeded partition nodes

mtdparts is passed from command line, so there is no need to have a
default partitioning in device-tree.

Suggested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>