ARM: dt: mvebu: ix4-300d: remove whole flash partition

Current NAND node has an additional flash partition for the whole
flash overlapping with real partitions. Remove this partition as
the whole flash is already represented by the NAND device itself.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>