rtc: cpcap: new rtc driver

This driver supports the Motorola CPCAP PMIC found on
some of Motorola's mobile phones, such as the Droid 4.

Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>