bnxt_en: Use non-standard attribute to expose shutdown temperature

Implement the sysfs attributes directly in the driver for
shutdown threshold temperature and pass an extra attribute group
to the hwmon core when registering the hwmon device.

Link: https://lore.kernel.org/netdev/20230815045658.80494-12-michael.chan@broadcom.com/


 Cc: Jean Delvare <jdelvare@suse.com>
 Cc: Guenter Roeck <linux@roeck-us.net>
 Cc: linux-hwmon@vger.kernel.org
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Signed-off-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>