target-arm/helper.c: Allow const opaques in arm CP

target-arm/helper.c: Allow const opaques in arm CP

Allow for defining const opaque data in ARM CP register definitions by
setting .opaque = foo. If non null opaque is passed into
define_one_arm_cp_reg_with_opaque then that opaque will take
precedence, otherwise if null opaque is passed, the original opaque
data will be used.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: cf0a3ac3438d97464240db9f5f4ef1585cbc1d77.1373429432.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>