ARM: highbank: Only touch common coherency control register fields

Midway adds new register fields to the coherency control registers, so
writing absolute values will break on Midway. Change the register
accesses to only modify the necessary and common fields in order to
support both Midway and Highbank.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Olof Johansson <olof@lixom.net>