powerpc: reserve fixmap entries for early debug

Add a set of entries to the fixmap table to allow usage of known
reserved virtual address space by early debug code.

The address space reserved is the top 128K of the 32-bit address
space. This allows, if required, the use of a BAT to do the mappings.

Signed-off-by: Albert Herranz <albert_herranz@yahoo.es>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>