hw/intc/armv7m_nvic: NS BFAR and BFSR are RAZ/WI if BFHFNMINS == 0

hw/intc/armv7m_nvic: NS BFAR and BFSR are RAZ/WI if BFHFNMINS == 0

The non-secure versions of the BFAR and BFSR registers are
supposed to be RAZ/WI if AICR.BFHFNMINS == 0; we were
incorrectly allowing NS code to access the real values.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190430131439.25251-3-peter.maydell@linaro.org