sysreset: syscon: add support for power off

The new type sysreset 'POWER_OFF', introduced by
commit 751fed42 ("sysreset: Add a way to find the last reset")
is only supported for "syscon-poweroff" compatible.

For details see Linux binding:
./Documentation/devicetree/bindings/power/reset/syscon-poweroff.txt

This patch removes the support of POWER_OFF for "syscon-reboot"
and keeps only the COLD reset (for command reset support)
and it introduces the compatible "syscon-poweroff"
for the POWER_OFF case.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>