[POWERPC] Put mpc52xx support file in platforms/52xx

platforms/embedded6xx is probably going away, and 52xx boards need
some extra support the 52xx interrupt controller and DMA engine
anyway.  It makes sense to group all the 52xx bits into a single path.

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