MIPS: move mips_{set,get}_machine_name() to a more generic place

Previously this functionality was only available to users of the mips_machine
api. Moving the code to prom.c allows us to also add a OF wrapper.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5164/