target-arm: Implement RVBAR register

target-arm: Implement RVBAR register

Implement the AArch64 RVBAR register, which indicates the reset
address. Since the reset address is implementation defined and
usually configurable by setting config signals in hardware, we
also provide a QOM property so it can be set at board level if
necessary.

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