PS3: Fix memory hotplug

[Benjamin Herrenschmidt]

> Paul, any thoughts here ? Should we add a lock ? That would mean being
> careful as the LMB stuff can be called very early, and spinlock wants
> things like PACA and possibly lockdep to be around.. 

Actually, we call early_init_devtree(), which populates the LMB, after
we initialize the PACA and lockdep, so it -should- be safe to add a
spinlock.

Dave ? Any objection to adding a spinlock to the LMB code ?

Cheers,
Ben.