[POWERPC] Export flat device tree via debugfs for debugging

If DEBUG is turned on in prom.c, export the flat device tree via debugfs.
This has been handy on several occasions.

To look at it:
 # mount -t debugfs none /sys/kernel/debug
 # od -a /sys/kernel/debug/powerpc/flat-device-tree
 and/or
 # dtc -fI dtb /sys/kernel/debug/powerpc/flat-device-tree -O dts

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>