cpuidle/pseries: Make symbol 'pseries_idle_driver' static

The sparse tool complains as follows:

drivers/cpuidle/cpuidle-pseries.c:25:23: warning:
 symbol 'pseries_idle_driver' was not declared. Should it be static?

'pseries_idle_driver' is not used outside of this file, so marks
it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200714142424.66648-1-weiyongjun1@huawei.com