Auto merge of #62 - hannobraun:update-nvic, r=japaric

Update NVIC

This pull request updated the NVIC definition and brings it in line with the ARMv7-M technical reference manual. Since ARMv6-M's NVIC is a subset of ARMv7-M's one, this should work on both platforms.

I tried adding some `#[cfg(armv7m]`, to make only the registers available on ARMv6-M visible on that platform, but aborted that plan, as it seemed to add a lot of complexity. What do you think about this, @japaric?

I also checked the [ARMv8-M Technical Reference Manual](https://static.docs.arm.com/ddi0553/a/DDI0553A_e_armv8m_arm.pdf). The NVIC is largely identical to ARMv7-M's one and only adds an additional block of registers between IABR and IPR, so it will be straight-forward to add support once that becomes relevant.