drivers/rtc/rtc-armada38x: add a new RTC driver for recent mvebu SoCs

The new mvebu SoCs come with a new RTC driver. This patch adds the
support for this new IP which is currently found in the Armada 38x
SoCs.

This RTC provides two alarms, but only the first one is used in the
driver. The RTC also allows using periodic interrupts.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reviewed-by: Arnaud Ebalard <arno@natisbad.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Boris BREZILLON <boris.brezillon@free-electrons.com>
Cc: Lior Amsalem <alior@marvell.com>
Cc: Tawfik Bayouk <tawfik@marvell.com>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>