ARM: oxnas: Add OX820 SMP support

The Oxford Semiconductor OX820 is a ARM11MPcore based SoC sharing some
features with the OX810 earlier SoC.
This patch adds the core to wake up the second core.

Clarifications about Copyrights dates :
- hotplug.c was taken from an old versatile code by Ma Haijun and left verbatim
- headsmp.S was taken from an old versatile code and adapted by Ma Haijun
- platsmp.c is a mix from versatile code, Ma Haijun code and my code for DT
Hence the 2002/2003 ARM Coryrights and 2013 Ma Haijun Copyrights.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>