drivers: wifi: siwx91x: Add support for set_power_save()

Added power-save mode support to optimize station wake-up timing
and reduce power consumption. The device currently supports
only legacy power-save mode.

This feature can be tested using the Wifi shell:
  - ps: Enable the PS. By default, the device operates in fast
        PSP mode.
  - ps_exit_strategy: Updates mode if enabled; otherwise, follows
        the configured exit strategy when power-save is enabled.
  - ps_wakeup_mode: Configures the wake-up behavior.
  - ps_timeout: Defines the timeout duration for power-save mode.

Co-authored-by: Arunmani Alagarsamy <arunmani.a@silabs.com>
Signed-off-by: Arunmani Alagarsamy <arunmani.a@silabs.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>