powerpc: Add A2 cpu support

Add the cputable entry, regs and setup & restore entries for
the PowerPC A2 core.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>