gpio/gpio-pl061: No need of thaw and poweroff routines for hibernate

pl061 uses same routines for suspend/freeze/poweroff and resume/thaw/restore.
We are only saving and restoring register values on these routines.

During hibernation, in freeze() we take a snapshot of gpio registers. In thaw()
we don't actually need to restore these registers, as power was never shut down
till now. Similarly, in poweroff() we don't need to take snapshot of these
registers again, as it was done during freeze() and by now the image is already
saved on disk.

This patch passes poweroff() and thaw() routines as NULL to avoid this extra
work done.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>