arm: spear6xx: switch spear600-evb to the new flash partition DT binding

This commit moves spear600-evb.dts to use the new flash partition Device
Tree binding documented in
Documentation/devicetree/bindings/mtd/partition.txt.

As this Device Tree binding document says: "For backwards compatibility
partitions as direct subnodes of the mtd device are supported. This use
is discouraged."

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>