FSL LAW: Keep track of LAW allocations

FSL LAW: Keep track of LAW allocations

Make it so we keep track of which LAWs have allocated and provide
a function (set_next_law) which can allocate a LAW for us if one is
free.

In the future we will move to doing more "dynamic" LAW allocation
since the majority of users dont really care about what LAW number
they are at.

Also, add CONFIG_MPC8540 or CONFIG_MPC8560 to those boards which needed them

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Andy Fleming <afleming@freescale.com>