power: axp: Avoid do_poweroff conflict with sysreset

The sysreset uclass has an option to provide the do_poweroff() function.
When that option is enabled, the AXP power drivers should not provide
their own definition.

For the AXP305, which is paired with 64-bit systems where TF-A provides
PSCI, there is another possible conflict with the PSCI firmware driver.
This driver can be enabled even if CONFIG_PSCI_RESET is disabled, so
make sure to use the right symbol in the condition.

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>