mips: use early_init_dt_scan

Convert mips to use new early_init_dt_scan function.

Remove early_init_dt_scan_memory_arch

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>