nvic: Disable the non-secure HardFault if AIRCR.BFHFNMINS is clear

nvic: Disable the non-secure HardFault if AIRCR.BFHFNMINS is clear

If AIRCR.BFHFNMINS is clear, then although NonSecure HardFault
can still be pended via SHCSR.HARDFAULTPENDED it mustn't actually
preempt execution. The simple way to achieve this is to clear the
enable bit for it, since the enable bit isn't guest visible.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1505240046-11454-15-git-send-email-peter.maydell@linaro.org