[POWERPC] Move Efika support files into platforms/52xx

The Efika board isn't different enough from other 52xx based boards to
justify a separate platform.  This patch merges it with the support
code for all other 52xx based boards.

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