powerpc/44x: 44x TLB doesn't need "Guarded" set for all pages

After discussing with chip designers, it appears that it's not
necessary to set G everywhere on 440 cores. The various core
errata related to prefetch should be sorted out by firmware by
disabling icache prefetching in CCR0. We add the workaround to
the kernel however just in case oooold firmwares don't do it.

This is valid for -all- 4xx core variants. Later ones hard wire
the absence of prefetch but it doesn't harm to clear the bits
in CCR0 (they should already be cleared anyway).

We still leave G=1 on the linear mapping for now, we need to
stop over-mapping RAM to be able to remove it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>