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.

pkgs/os-specific/linux/kernel/common-config.nix

@@ -52,6 +52,9 @@ let
       CPU_FREQ_DEFAULT_GOV_PERFORMANCE = yes;
       CPU_FREQ_GOV_SCHEDUTIL           = whenAtLeast "4.9" yes;
       PM_WAKELOCKS                     = yes;
+      # Power-capping framework and support for INTEL RAPL
+      POWERCAP                         = yes;
+      INTEL_RAPL                       = module;
     };
 
     external-firmware = {