metag: use common of_flat_dt_match_machine

Convert metag to use the common of_flat_dt_get_machine_name function.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
[james.hogan: fix missing arch_get_next_mach and const mismatch]
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: James Hogan <james.hogan@imgtec.com>