rtc: armada38x: add the device tree binding documentation

The Marvell Armada 38x SoCs contains an RTC which differs from the RTC
used in the other mvebu SoCs until now.  This forth version of the patch
set adds support for this new IP and enable it in the Device Tree of the
Armada 38x SoC.

This patch (of 5):

The Armada 38x SoCs come with a new RTC which differs from the one used in
the other mvebu SoCs until now.  This patch describes the binding of this
RTC.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
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: Arnaud Ebalard <arno@natisbad.org>
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>