arm: add documentation describing Marvell families of SoC

As stated in the introduction of the document, the families of ARM
SoCs at Marvell are very complicated, and it is difficult for
newcomers to understand the organization of this SoC family and how it
relates to the Linux kernel support for those hardware platforms.

This document is only at RFC stage for now, it requires reviews and
comments from the Marvell maintainers, the PXA maintainers and the MMP
maintainers. For correctness of course, but also to add any other
information that would be useful. For example, one of the thing that
wasn't clear how to detail in the documentation is how the SoCs relate
to each other in terms of hardware IP blocks. For example, most of the
Kirkwood/Dove/Armada 370-XP/etc. hardware IPs (I2C, SPI, USB, SATA,
etc.) are identical, while the PXA and MMP families are completely
separate.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Gregory Clement <gregory.clement@free-electrons.com>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Olof Johansson <olof@lixom.net>
Cc: Lior Amsalem <alior@marvell.com>
Cc: Maen Suleiman <maen@marvell.com>
Cc: Tawfik Bayouk <tawfik@marvell.com>
Cc: Shadi Ammouri <shadi@marvell.com>
Cc: Eran Ben-Avi <benavi@marvell.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>