Lite5200 shouldn't mess with ROOT_DEV

There is no good reason for board platform code to mess with the ROOT_DEV.
Remove it from all in-tree platforms except powermac

This is a follow on to commit 745e1027

.
The original patch had this change to lite5200.c, but it got dropped in
the psycho madness that is the 2.6.24 merge window.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>