sysreset: switch to using SYSRESET_POWER_OFF for poweroff

It seems that SYSRESET_POWER_OFF was added recently, and all previous code
used SYSRESET_POWER for poweroff. SYSRESET_POWER is supposed to be a
PMIC-level power cycle, not a poweroff.

(Comment by Simon Glass)
SYSRESET_POWER means to do a power reset (removing and reinstating all power)
SYSRESET_POWER_OFF means to turn the device off and leave it off

Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
(Update comment to help understand the patch)
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>