of: Increase MAX_PHANDLE_ARGS

arm-smmu driver uses of_parse_phandle_with_args when parsing DT
information to determine stream IDs for a master device.
Thus the number of stream IDs per master device is bound by
MAX_PHANDLE_ARGS.

To support Calxeda ECX-2000 hardware arm-smmu driver requires a
slightly higher value for MAX_PHANDLE_ARGS as this hardware has 10
stream IDs for one master device.

Increasing it to 16 seems a reasonable choice.

Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Cc: Andreas Herrmann <herrmann.der.user@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Herrmann <andreas.herrmann@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>