spmi: Fix compilation error when CONFIG_PM_RUNTIME is defined

<linux/pm_runtime.h> should be included to fix any compilation errors
associated with enabling the PM_RUNTIME config.

Change-Id: Ided562e1351ccb5eef8957c305060275a5679d42
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>