of/fdt: create common debugfs

Both powerpc and microblaze have the same FDT blob in debugfs feature.
Move this to common location and remove the powerpc and microblaze
implementations. This feature could become more useful when FDT
overlay support is added.

This changes the path of the blob from "$arch/flat-device-tree" to
"device-tree/flat-device-tree".

Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
Tested-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Stephen Chivers <schivers@csc.com>