arch/soc: introduce config for custom arch_cpu_idle implementation

Each arch platform may has a general arch_cpu_idle implementation but
each vendor may has a custom one, so this config will be used for vendor
to override it.

Some workarounds were introduced for intel cavs2.5 platform bring up.
It is not general so move them to platform code.

Signed-off-by: Rander Wang <rander.wang@intel.com>