dt/bindings: Remove device_type "serial" from marvell,mv64360-mpsc

device_type is deprecated. There is no need to check for it in device
driver code and no need to specify it in the device tree. Remove the
property from stock .dts files and remove the check for it from device
drivers. This change should be 100% backwards compatible with old device
trees.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>