target-arm/helper.c: Allow cp15.c15 dummy override

The cp15.c15 space is implementation defined. Currently there is a
dummy placeholder register RAZing it. Allow overriding of this RAZ
so implementations of specific registers can take precedence.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: ed1bacec56dae00cb398c798f8240e8e685f949c.1387160489.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
