target-arm: Implement NSACR trapping behaviour

target-arm: Implement NSACR trapping behaviour

Implement some corner cases of the behaviour of the NSACR
register on ARMv8:
 * if EL3 is AArch64 then accessing the NSACR from Secure EL1
   with AArch32 should trap to EL3
 * if EL3 is not present or is AArch64 then reads from NS EL1 and
   NS EL2 return constant 0xc00

It would in theory be possible to implement all these with
a single reginfo definition, but for clarity we use three
separate definitions for the three cases and install the
right one based on the CPU feature flags.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1454506721-11843-7-git-send-email-peter.maydell@linaro.org