ppc/85xx: Add tracking of TLB CAM usage

We need to track which TLB CAM entries are used to allow us to
"dynamically" allocate entries later in the code.  For example the SPD
DDR code today hard codes which TLB entries it uses.  We can now make
that pick entries that are free.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>