mips: dts: Fix missing device_type="memory" property in memory nodes

A few platforms lack a 'device_type = "memory"' for their memory
nodes, relying on an old ppc quirk in order to discover its memory.
Add the missing data so that all parsing code can find memory nodes
correctly.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: <stable@vger.kernel.org>
Acked-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Grant Likely <grant.likely@linaro.org>