RTC: add DT bindings to pxa-rtc

This patch adds generic device tree bindings to the PXA RTC driver.
Documentation for bindings were also added.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>