dt-bindings: rtc: add stericsson,coh901331 bindings

Add device tree bindings for the ST-Ericsson COH 901 331 Real Time Clock

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Rob Herring <robh@kernel.org>