Create a virtual PHY peripheral, to handle initialization.

[Frostie314159]

Currently, there is no virtual peripheral for the PHY. This is fine, as long as only one module is in use, since register_chipv7_phy will only be called once. If for example, esp-wifi and esp-ieee802154 were to be used simultaneously (through some currently not existing coex mechanism), register_chipv7_phy might be called more than once.
I think, that having such a peripheral would be nice to have, and clean up a lot of code in esp-wifi.