MIPS: Remove empty prom_free_prom_memory functions

Most of the prom_free_prom_memory functions are empty. With
a new weak prom_free_prom_memory() we can remove all of them.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>