ARM: highbank: add reset support for Calxeda Midway machine

The Calxeda Midway part has A15 cores, which do not have the Highbank
A9's SCU used there for resetting the chip.
Add code to distinguish between the A9 and the A15 and invoke the
appropriate register writes to support the newer part.

Andre: rework detection of Highbank vs. Midway
Rob: fix Andre's reworked detection

Signed-off-by: Mark Langsdorf <mark.langsdorf@gmail.com>
Signed-off-by: Andre Przywara <osp@andrep.de>
Signed-off-by: Rob Herring <robh@kernel.org>