rtc: st: Add new driver for ST's LPC RTC

ST's Low Power Controller (LPC) controls two devices; watchdog and RTC.
Only one of the devices can be used at any one time.  This is enforced
by the correlating MFD driver.  This portion of the driver-set controls
the Real Time Clock.

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