stellaris: Don't hw_error() on bad register accesses

Current recommended style is to log a guest error on bad register
accesses, not kill the whole system with hw_error().  Change the
hw_error() calls to log as LOG_GUEST_ERROR or LOG_UNIMP or use
g_assert_not_reached() as appropriate.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 1491486314-25823-1-git-send-email-peter.maydell@linaro.org