New support for ARM Cortex-M7 in v1.8.x

[jnaulet]

A new architecture is staging in v1.8.x, ARM Cortex-M7.
This is more or less a Cortex-M4F with caches, so it's quite an easy task.

As a demo board, the DevEBox STM32H7XX with a STM32H743VIT6 is available in the staging tree, too.
For the moment, it boots fine but there's only partial support for clocks, mux and gpios.