mtd: spear_smi: add CONFIG_PM_SLEEP to suspend/resume functions

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. Also, unnecessary CONFIG_PM ifdefs
are removed

drivers/mtd/devices/spear_smi.c:1049:12: warning: 'spear_smi_suspend' defined but not used [-Wunused-function]
drivers/mtd/devices/spear_smi.c:1059:12: warning: 'spear_smi_resume' defined but not used [-Wunused-function]

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>