rtc: add support for DS3232 device

DS3232 is an i2c RTC with 236 bytes of battery-backed SRAM.

Add an RTC driver for DS3232 device, which provides time and
date support. Also read and write functions are provided,
which can be used to access the SRAM memory.

Signed-off-by: Nandor Han <nandor.han@vaisala.com>