[POWERPC] 52xx: Don't use device_initcall to probe of_platform_bus

Using device_initcall makes it happen for every platform that
compiles this file in. This is really bad, for obvious reasons.

Instead, we use the .init field of the machine description. If
the platform needs the hook to do something specific it can provides
its own function and call mpc52xx_declare_of_platform_devices from
there. If not, the mpc52xx_declare_of_platform_devices function can
directly be used as the init hook.

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