rtc: rtc-lpc32xx: introduce RTC driver for the LPC32XX SoC

Add an RTC driver for the built-in RTC in the LPC32XX SoC.  This patch
includes updates from the initial review comments and updates from the v3
review.

Signed-off-by: Kevin Wells <wellsk40@gmail.com>
Signed-off-by: Durgesh Pattamatta <durgesh.pattamatta@nxp.com>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>