sunxi: Use sysreset framework for poweroff/reset

Instead of hardcoding the watchdog for reset, and the PMIC for poweroff,
use the sysreset framework to manage the available poweroff/reset
backends. This allows (as examples) using the PMIC to do a cold reset,
and using a GPIO to power off H3/H5 boards lacking a PMIC. Furthermore,
it removes the need to hardcode watchdog MMIO addresses, since the
sysreset backends can be discovered using the device tree.

Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Stefan Roese <sr@denx.de>