target/arm: Implement NSACR gating of floating point

target/arm: Implement NSACR gating of floating point

The NSACR register allows secure code to configure the FPU
to be inaccessible to non-secure code. If the NSACR.CP10
bit is set then:
 * NS accesses to the FPU trap as UNDEF (ie to NS EL1 or EL2)
 * CPACR.{CP10,CP11} behave as if RAZ/WI
 * HCPTR.{TCP11,TCP10} behave as if RAO/WI

Note that we do not implement the NSACR.NSASEDIS bit which
gates only access to Advanced SIMD, in the same way that
we don't implement the equivalent CPACR.ASEDIS and HCPTR.TASE.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20190510110357.18825-1-peter.maydell@linaro.org