Depend on upstream version of cortex-m

[hannobraun]

We're currently depending on my fork of cortex-m, as the upstream version won't build correctly for us. I've submitted a pull request.

[hannobraun]

A fix for the issue has been merged upstream, but it's been released together with some breaking changes that I'm not yet ready for. Since we don't depend directly on my cortex-m fork, instead [patch]ing it in Cargo.toml, it should be fine to leave this as is for the time being.

This issue will be properly fixed later, when we upgrade to cortex-m 0.4.x. This is going to be part of a larger effort, as other changes need to make their way downstream from svd2rust via lpc82x at the same time.