Add boot-default ClockConfig

[bjoernQ]

This is the first part of #44

While it doesn't allow you to change clock rates, yet it prepares for this by introducing ClockControll::boot_defaults()

• there is a i2c_clock which doesn't exist - it's because according to the TRM on ESP32 it's always 80MHz and it's not clear to me from where that is ... so, adding this for now seems to be the easiest solution - not ideal but should be okay for now
• the struct only contains clocks we actually use currently - if we need more, we should add them as needed
• also, this adds system which splits the SYSTEM/DPORT - currently only into PeripheralClockControl and SystemClockControl
• there is nothing yet for SystemClockControl - will be added once we actually allow to change clocks

I think this is an improvement in general - definitely more to come however - and I guess this contains enough changes for a single PR

[MabezDev]

Looking good to me! PeripheralClockControl is nice, a lot let cfg_if's sprinkled around now - all in one place for the most part.

[jessebraham]

What is the purpose of this field?

[MabezDev]

Without this, the end user would be able to construct Clocks from thin air. This private field means it can only come from the HAL.