mtd: spear_smi: Move suspend/resume to follow dev_pm_ops

Use dev_pm_ops to support PM specific callbacks.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>