pseries/cpuidle: Remove dependency of pseries.h file

As a part of pseries_idle cleanup to make the backend driver
code common to both pseries and powernv.
Remove non-essential smt_snooze_delay declaration in pseries.h
header file and pseries.h file inclusion in
pseries/processor_idle.c

Signed-off-by: Deepthi Dharwar <deepthi@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>