rtc: Add support for Intersil ISL12057 I2C RTC chip

Intersil ISL12057 is an I2C RTC chip also supporting two alarms. This
patch only adds support for basic RTC functionalities (i.e. getting
and setting time). Tests have been performed on NETGEAR ReadyNAS 102
w/ startup/shutdown scripts, hwclock, ntpdate and openntpd.

Reviewed-by: Mark Brown <broonie@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>