Add CONFIG_PM_SLEEP to suspend/resume functions to fix the following
build warning when CONFIG_PM_SLEEP is not selected. This is because
sleep PM callbacks defined by SIMPLE_DEV_PM_OPS are only used when
the CONFIG_PM_SLEEP is enabled.

drivers/block/nvme-core.c:2541:12: warning: 'nvme_suspend' defined but not used [-Wunused-function]
drivers/block/nvme-core.c:2550:12: warning: 'nvme_resume' defined but not used [-Wunused-function]
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>