nvic: Implement v8M changes to fixed priority exceptions

nvic: Implement v8M changes to fixed priority exceptions

In v7M, the fixed-priority exceptions are:
 Reset: -3
 NMI: -2
 HardFault: -1

In v8M, this changes because Secure HardFault may need
to be prioritised above NMI:
 Reset: -4
 Secure HardFault if AIRCR.BFHFNMINS == 1: -3
 NMI: -2
 Secure HardFault if AIRCR.BFHFNMINS == 0: -1
 NonSecure HardFault: -1

Make these changes, including support for changing the
priority of Secure HardFault as AIRCR.BFHFNMINS changes.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1505240046-11454-14-git-send-email-peter.maydell@linaro.org