rtc: add initial support for mcp7941x parts

Add initial support for the microchip mcp7941x series of real time clocks.

The mcp7941x series is generally compatible with the ds1307 and ds1337 rtc
devices from dallas semiconductor.  minor differences include a backup
battery enable bit, and the polarity of the oscillator enable bit.

Signed-off-by: David Anders <danders.dev@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>