of/platform: disable the of_platform_default_populate_init() for all the ppc boards

With the commit 44a7185c ("of/platform: Add common method to
populate default bus"), a default function is introduced to populate
the default bus and this function is invoked at the arch_initcall_sync
level. But a lot of ppc boards use machine_device_initcall() to
populate the default bus. This means that the default populate function
has higher priority and would override the arch specific population of
the bus. The side effect is that some arch specific bus are not probed,
then cause various malfunction due to the miss of some devices. Since
it is very possible to introduce bugs if we simply change the initcall
level for all these boards(about 30+). This just disable this default
function for all the ppc boards.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>