ARM: highbank: abstract out SCU usage

In preparation for A15 support on ECX-2000, the direct calls to SCU
registers must be conditional. The SCU power mode register is replaced by
a custom register on ECX-2000.

Rather than read the number of cores from the SCU, just hardcode it to 4.
This removes one use of SCU and removes the need for the SCU to be
statically mapped. The cpu initialization will ultimately come from DT.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>