powerpc: Fix goof in 6xx and POWER4 idle power-save functions

This fixes a mistake I made when editing these functions - when I
took out the interrupt disabling code (because interrupts are now
disabled by the caller) I left the register that is used for the MSR
value to be used during doze/nap uninitialized.  This fixes it.

Also updated some of the comments in idle_power4.S and removed some
code that was copied over from idle_6xx.S but is no longer relevant
(we don't ever clear the CPU_FTR_CAN_NAP bit at runtime for POWER4).

Signed-off-by: Paul Mackerras <paulus@samba.org>