bnxt_en: Move hwmon functions into a dedicated file

This is in preparation for upcoming patches in the series.
Driver has to expose more threshold temperatures through the
hwmon sysfs interface. More code will be added and do not
want to overload bnxt.c.

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