mfd: rn5t618: Register restart handler

Use the PMIC's repower capability for reboots. Register a restart
handler and use a slightly elevated priority of 192 since the PMIC
has suprior reset capability (causing a system wide reset).

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>