power: reset: pwr-mlxbf: support graceful shutdown

power: reset: pwr-mlxbf: support graceful shutdown

The OCP board used a BlueField's GPIO pin for entering
low power mode. That board was not commercialized and
has been dropped from production so all its code is unused.
The new hardware requirement is to trigger a graceful shutdown
when that GPIO pin is toggled. So replace the unused low power
mode with a graceful shutdown.

Signed-off-by: Asmaa Mnebhi <asmaa@nvidia.com>
Reviewed-by: David Thompson <davthompson@nvidia.com>
Link: https://lore.kernel.org/r/20240611134327.30975-1-asmaa@nvidia.com
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>