target/arm: Implement M-profile "minimal RAS implementation"

For v8.1M the architecture mandates that CPUs must provide at
least the "minimal RAS implementation" from the Reliability,
Availability and Serviceability extension. This consists of:
 * an ESB instruction which is a NOP
   -- since it is in the HINT space we need only add a comment
 * an RFSR register which will RAZ/WI
 * a RAZ/WI AIRCR.IESB bit
   -- the code which handles writes to AIRCR does not allow setting
      of RES0 bits, so we already treat this as RAZ/WI; add a comment
      noting that this is deliberate
 * minimal implementation of the RAS register block at 0xe0005000
   -- this will be in a subsequent commit
 * setting the ID_PFR0.RAS field to 0b0010
   -- we will do this when we add the Cortex-M55 CPU model

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201119215617.29887-26-peter.maydell@linaro.org

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