target-arm: Implement AArch64 dummy breakpoint and watchpoint registers

target-arm: Implement AArch64 dummy breakpoint and watchpoint registers

In AArch64 the breakpoint and watchpoint registers are mandatory, so the
kernel always accesses them on bootup. Implement dummy versions, which
read as written but have no actual effect.

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