target-arm: Implement minimal DBGVCR, OSDLR_EL1, MDCCSR_EL0

Implement debug registers DBGVCR, OSDLR_EL1 and MDCCSR_EL0
(as dummy or limited-functionality). 32 bit Linux kernels will
access these at startup so they are required for breakpoints
and watchpoints to be supported.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
