target-arm: Add checks that cpreg raw accesses are handled

Add assertion checking when cpreg structures are registered that they
either forbid raw-access attempts or at least make an attempt at
handling them. Also add an assert in the raw-accessor-of-last-resort,
to avoid silently doing a read or write from offset zero, which is
actually AArch32 CPU register r0.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1422282372-13735-3-git-send-email-peter.maydell@linaro.org
Reviewed-by: Greg Bellows <greg.bellows@linaro.org>
