85xx: Update multicore boot mechanism to ePAPR v0.81 spec

The following changes are needed to be inline with ePAPR v0.81:

* r4, r5 and now always set to 0 on boot release
* r7 is used to pass the size of the initial map area (IMA)
* EPAPR_MAGIC value changed for book-e processors
* changes in the spin table layout
* spin table supports a 64-bit physical release address

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