hw/intc/armv7m_nvic: Implement read/write for RAS register block

hw/intc/armv7m_nvic: Implement read/write for RAS register block

The RAS feature has a block of memory-mapped registers at offset
0x5000 within the PPB.  For a "minimal RAS" implementation we provide
no error records and so the only registers that exist in the block
are ERRIIDR and ERRDEVID.

The "RAZ/WI for privileged, BusFault for nonprivileged" behaviour
of the "nvic-default" region is actually valid for minimal-RAS,
so the main benefit of providing an explicit implementation of
the register block is more accurate LOG_UNIMP messages, and a
framework for where we could add a real RAS implementation later
if necessary.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201119215617.29887-27-peter.maydell@linaro.org