hw/intc/armv7m_nvic: Fix ICSR PENDNMISET/CLR handling

hw/intc/armv7m_nvic: Fix ICSR PENDNMISET/CLR handling

The PENDNMISET/CLR bits in the ICSR should be RAZ/WI from
NonSecure state if the AIRCR.BFHFNMINS bit is zero. We had
misimplemented this as making the bits RAZ/WI from both
Secure and NonSecure states. Fix this bug by checking
attrs.secure so that Secure code can pend and unpend NMIs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180209165810.6668-3-peter.maydell@linaro.org