ch states that the XScale core clock and power management
registers aren't implemented on ixp42x [3.5.2.2].

Also, the disabled code has suffered from bit rot:
- it checks hlt_counter which is obsolete, as that variable
  and all related code now is private to kernel/process.c
- it passes too many parameters to cpu_do_idle()

So this patch:
- adds a comment before the #if 0 to explain why
  cpu_do_idle() mustn't be called on ixp4xx
- removes the obsolete test of hlt_counter and the
  obsolete parameter to cpu_do_idle()

This is purely a documentation fixup and changes no
generated code. Even so, it has been tested on an
ixp420 machine (ds101).

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>
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