powerpc: Move duplicated BAT defines to mmu.h

The BAT fields are architected; there's no need for these to be in
cpu-specific files.  Drop the duplication and move these to
include/asm-ppc/mmu.h.  Also, remove the BL_xxx defines that were only
used by the alaska board, and switch to using the BATU_BL_xxx defines
used by all the other boards.  The BL_ defines previously in use
had to be shifted into the proper position for use, which was inefficient.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>