the annotation of prom_early_alloc is wrong.

prom_early_alloc() is annotated __init, and users of
kernel_tree_alloc() is also annotated __init.
So simply match the annoation of these to fix the warning.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Pôdõ•"x