powerpc/mpc5121: avoid using arch_initcall for clock init

Move mpc5121_clk_init() call to platform init code so it won't
get called on non-5121 platforms on a multiplatform kernel.

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