ARM: highbank: call highbank_pm_init from .init_machine

Being a module_init call, highbank_pm_init will cause problem with
multi-platform build running on other platforms.  Call it from
.init_machine instead.

Reported-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Olof Johansson <olof@lixom.net>