powerpc/5200: convert mpc5200 to use of_platform_populate()

of_platform_populate() also handles nodes at the root of the tree,
which is wanted for things like describing the sound complex.  This
patch converts mpc5200 support to use of_platform_populate() instead
of of_platform_bus_probe().

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Anatolij Gustschin <agust@denx.de>