nvic: Implement Security Attribution Unit registers

Implement the register interface for the SAU: SAU_CTRL,
SAU_TYPE, SAU_RNR, SAU_RBAR and SAU_RLAR. None of the
actual behaviour is implemented here; registers just
read back as written.

When the CPU definition for Cortex-M33 is eventually
added, its initfn will set cpu->sau_sregion, in the same
way that we currently set cpu->pmsav7_dregion for the
M3 and M4.

Number of SAU regions is typically a configurable
CPU parameter, but this patch doesn't provide a
QEMU CPU property for it. We can easily add one when
we have a board that requires it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1506092407-26985-14-git-send-email-peter.maydell@linaro.org
