sysreset: move stm32mp sysreset poweroff implementation to sysreset uclass

This is a generic implementation. Add CONFIG_SYSRESET_CMD_POWEROFF
to signal when we need it. Enable it from the STPMIC1 config and in
sandbox.

The config flag is transitionary, that is it can be removed after all
poweroff implementations use sysreset, and just have CMD_POWEROFF depend
on sysreset.

Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Tested-by: Patrick Delaunay <patrick.delaunay@st.com>