mfd: dt-bindings: twl4030-power: Use the standard property to mark power control

ti,system-power-controller is more or less the standard way of
indicating that the PMIC is the system wide power controller and hence
may be used to switch off the system. Almost ALL TI PMIC drivers and
many Maxim PMIC drivers follow the same style.

So support 'ti,system-power-controller' in addition to the usual
'ti,use_poweroff' to indicate that the PMIC instance has control for
switching off the system.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>