firmware: ti_sci: Add support for reboot core service

Since system controller now has control over SoC power management, it
needs to be explicitly requested to reboot the SoC. Add support for
it.

In some systems however, SoC needs to toggle a GPIO or send event to an
external entity (like a PMIC) for a system reboot to take place. To
facilitate that, we allow for a DT property to determine if the reboot
handler will be registered and further, the service is also made
available to other drivers (such as PMIC driver) to sequence the
additional operation and trigger the SoC reboot as the last step.

Tested-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>