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>

---
Changes from v3:
 * Add Arnd Bergmann Acked-by.

Changes from v2:
 * Mention the plat-<foo> directory that each SoC family is using.
 * Take into account Eric Miao's comments on the PXA/MMP families: add
   PXA21x/PXA25x/PXA26x to the list, mention which processors are
   AP+CP or AP only. Clarify the comments on which SoCs were designed
   by Intel, which ones were designed by Marvell after the acquisition
   of the XScale family.
 * Mention long-term plans regarding the support of those SoC
   families.

Changes from v1:
 * Added publicly available datasheet for the 88F5182 pointed by
   Andrew Lunn.
 * Added publicly available datasheet for the 88F5281 found with
   Google searches
 * Mentionned the Feroceon core name where appropriate, and Sheeva
   where appropriate
 * Fixed the core names for PXA930, PXA935 and PXA955 after comments
   from Arnd Bergmann. Wikipedia is mistakenly suggesting that PXA93x
   are Sheeva-based, and there isn't much information available on the
   web about the 955.