drivers: wifi: siwx91x: Add support for Enhanced Legacy Power Save

Siwx91x support a specific mode slightly better than the old legacy PS
mode.

This mode has to be set on the NWP boot, so it is not easy to configure
it during the runtime. Hence, this patch only provide a compile time
option to enable it.

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>