power: pmic: axp: Implement poweroff via sysreset

The AXP PMICs have the ability to power off the system. The existing
code for this is duplicated for each PMIC variant, and uses the legacy
non-DM "pmic_bus" interface. When SYSRESET is enabled, this can all be
replaced with a sysreset device using the DM_PMIC interface.

Since the trigger bit is the same on all PMIC variants, use the register
definitions from the oldest supported PMIC.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>