drivers/rtc: add support for Microchip MCP795

Add driver for SPI RTC Microchip MCP795.  Only supports saving/loading
time from the chip (i.  e.  no alarms/power events/ID).

Signed-off-by: Josef Gajdusek <atx@atx.name>
Cc: Jingoo Han <jg1.han@samsung.com>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>