[POWERPC] Make it blatantly clear; mpc5200 device tree is not yet stable

Documentation-only change.  The 5200 device tree layout has not yet
stablized, so nobody should depend on the layout of the tree.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>