powerpc: Pull out MPC106 (grackle) initialization code into its own file

This is so that the 32-bit CHRP code can use it.  The MPC106
initialization code is now in arch/powerpc/sysdev/grackle.c and
is controlled by CONFIG_PPC_MPC106.

Signed-off-by: Paul Mackerras <paulus@samba.org>