mips: use common of_flat_dt_get_machine_name

Convert mips to use the common of_flat_dt_get_machine_name function.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Acked-by: John Crispin <blogic@openwrt.org>