hvf: arm: Raise an exception for sysreg by default

Any sysreg access results in an exception unless defined otherwise so
we should raise an exception by default.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>