Add a driver for the Maxim DS3231 a low-cost, extremely accurate (+-2PPM)
I2C real-time clock (RTC).

The DS3231 has an integrated temperature-compensated crystal oscillator
(TXCO) and crystal.

DS3231 has a temperature sensor, an independent 32kHz output (which can be
turned on and off by the driver) and another output that can be used as
interrupt for alarms or as a second square-wave output, which frequency and
operation mode can be set by driver sysctl(8) knobs.

Differential Revision: https://reviews.freebsd.org/D1016
Reviewed by: ian, rpaulo
Tested on: Raspberry pi model B