ARM: sunxi: Reintroduce the restart code for A10/A20 SoCs

This partly reverts commits 55360050 (ARM: sunxi: Remove reset code from
the platform) and 5e669ec5 (ARM: sunxi: Remove init_machine callback) for
the sun4i, sun5i and sun7i families.

This is needed because the watchdog counterpart of these commits was dropped,
and didn't make it into 3.16. In order to still be able to reboot the board, we
need to reintroduce that code. Of course, the long term view is still to get
rid of that code in mach-sunxi.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Olof Johansson <olof@lixom.net>