kernel: set POWERCAP and INTEL_RAPL
kernel: set POWERCAP and INTEL_RAPL

These enable some power management settings. They are needed for [s-tui
to show power readings](https://github.com/amanusk/s-tui/issues/105).

The values chosen here match what Arch Linux has. In particular
the Intel specific code is loadable as a module rather than compiled in.

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