Both psci_system_reset and psci_system_off call psci_ops->system_reset,
which seems like a copy-paste error as there is an unused ->system_off
as well.

Adjust psci_system_off to use ->system_off instead. This won't matter
for the existing i.MX7 PSCI implementation because it defines neither
callback, but it will for the upcoming STM32MP PSCI support.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>