rtc: bindings: Provide ST bindings for ST's LPC RTC device

On current ST platforms the LPC controls a number of functions including
Watchdog and Real Time Clock.  This patch provides the bindings used to
configure LPC in RTC mode.

Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Lee Jones <lee.jones@linaro.org>