target/arm: Define an IDAU interface

In v8M, the Implementation Defined Attribution Unit (IDAU) is
a small piece of hardware typically implemented in the SoC
which provides board or SoC specific security attribution
information for each address that the CPU performs MPU/SAU
checks on. For QEMU, we model this with a QOM interface which
is implemented by the board or SoC object and connected to
the CPU using a link property.

This commit defines the new interface class, adds the link
property to the CPU object, and makes the SAU checking
code call the IDAU interface if one is present.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180220180325.29818-5-peter.maydell@linaro.org