target-arm: Implement XScale cache lockdown operations as NOPs

XScale defines some implementation-specific coprocessor registers
for doing cache lockdown operations. Since QEMU doesn't model a
cache no proper implementation is possible, but NOP out the
registers so that guest code like u-boot that tries to use them
doesn't crash.

Reported-by: <prqek@centrum.cz>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>