* Unify the system peripheral
Whilst the PCR, SYSTEM and DPORT peripherals are different, we currently
use them all in the same way. This PR unifies the peripheral name in the
hal to `SYSTEM`. The idea is that they all do the same sort of thing, so
we can collect them under the same name, and later down the line we can
being to expose differences under an extended API.
The benifits to this are imo quite big, the examples now are all identical,
which makes things easier for esp-wifi, and paves a path towards the
multichip hal.
Why not do this in the PAC? Imo the pac should be as close to the
hardware as possible, and the HAL is where we should abstractions such
as this.
* changelog
