Documentation: powerpc/fsl: Update compatible for l2cache binding

List all the current valid compatible strings for the l2cache binding.
This should stop checkpatch.pl from complaining and will hopefully save
someone from having to debug a typo in their dts.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>