drivers/rtc/rtc-ds1307.c: add alarm support for mcp7941x chips

Add alarm support for the Microchip RTC devices MCP794xx.  Note that two
programmable alarms are provided by the chip but only one is used by the
driver.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Gregory Clement <gregory.clement@free-electrons.com>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>