rtc: add rtc-lpc24xx driver

Add driver for the RTC found on NXP LPC178x/18xx/408x/43xx devices.
The RTC provides calendar and clock functionality together with
alarm interrupt support.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>