powerpc: fix linkstation and storcenter compilation breakage

Defining flash partition table in platform code is deprecated, and due to
recent changes linkstation and storcenter do not compile any more with
their default configurations because of undefined references to
physmap_set_partitions(). Instead of fixing them by using the correct
kernel configuration macro in preprocessor conditional, remove partition
table definitions altogether. Instead add support for partition definition
on the command-line and in device tree to the default configurations.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>