of/mips: Add device tree support to MIPS

Add the ability to enable CONFIG_OF on the MIPS architecture.

Signed-off-by: Dezhong Diao <dediao@cisco.com>
[grant.likely@secretlab.ca: cleared out obsolete hooks,
	removed ARCH_HAS_DEVTREE_MEM,
	remove __init tags from header file,
	removed debugfs support hunk]
[ddaney@linux-mips.org: backed out over aggressive trimming of hooks]
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Tested-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>