Adds support for SYSRESET_POWER_OFF = PMIC power off used by command
power off and introduced by commit 751fed426f ("sysreset: Add a way
to find the last reset").
The driver use SYSRESET_POWER for the PMIC-level power cycle, with restart.
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>