PATCH: improved processor config for G3s

Hi Paul,

It's patch day today ;-)

Attached is my try at improving the processor configuration for G3/G4
processors in head.S. Instead of leaving some config bits at what they
were set by the formware, I've tried to set all availbale faetures to
their 'optimum' value. Patch is against your 2.4 rsync tree as of today.
This patch, as well as the same thing against a 2.2 source, are also
posted on sourceforge.

Those where I see no problem are:

 * Branch History Table (BHTE), Branch Target ICache (BTIC), Dynamic
   Power Management (DPM) and Store Gathering (SGE) all on,
 * Instruction Cache Throttling Control (ICTC) off.

These were not touched before, but might be worth a discussion:

 * Speculative Cache Access Disable (SPD) cleared, Address Brodcast
   (ABE) enabled

Clearing SPD makes sense since some processor upgrade cards come with an
OF patch that sets this bit (due to ROM problems... we don't care once
Linux is up).

I'm running these processor settings without a problem for some time
now. If all the processor config in head.S doesn't look clean to some, I
agree, and I propose we change that setup a bit for 2.5, separating the
cache enable stuff from the processor configuration.

Cheers

Michel

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