mfd: tps65219: Make poweroff handler conditional on system-power-controller

mfd: tps65219: Make poweroff handler conditional on system-power-controller

Currently, the TPS65219 driver unconditionally registers a poweroff
handler. This causes issues on systems where a different component
(such as TF-A firmware) should handle system poweroff instead.

Make the poweroff handler registration conditional based on the
"system-power-controller" device tree property. This follows the
standard kernel pattern where only the designated power controller
registers for system poweroff operations.

On systems where the property is absent, the PMIC will not register
a poweroff handler, allowing other poweroff mechanisms to function.

Signed-off-by: Akashdeep Kaur <a-kaur@ti.com>
Link: https://patch.msgid.link/20260401112257.1248437-3-a-kaur@ti.com
Signed-off-by: Lee Jones <lee@kernel.org>