hw/misc: New "unimplemented" sysbus device

Create a new "unimplemented" sysbus device, which simply accepts
all read and write accesses, and implements them as read-as-zero,
write-ignored, with logging of the access as LOG_UNIMP.

This is useful for stubbing out bits of an SoC or board model
which haven't been written yet.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1484247815-15279-3-git-send-email-peter.maydell@linaro.org