Add driver for STMicroelectronics M41T60 RTC

This driver is based on the driver for the M41T11.  In the intended
application, the RTC will be powered by a large capacitor, rather than a
battery.  The driver therefore checks to see whether the RTC has lost
power.  The chip's OUT bit is normally reset from its power-up state.  If
the OUT bit is read as set, or if the date and time are not valid, then the
RTC is assumed to have lost power, and its date and time are reset to
1900-01-01 00:00:00.

Support for adjusting the speed of the clock to improve accuracy is
provided through an environment variable.

Signed-off-by: Larry Johnson <lrj@acm.org>