highbank: validate register offset before access

An 'offset' parameter sent to highbank register r/w functions
could be greater than number(NUM_REGS=0x200) of hb registers,
leading to an OOB access issue. Add check to avoid it.

Reported-by: Moguofang (Dennis mo) <moguofang@huawei.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-id: 20171113062658.9697-1-ppandit@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
