powerpc: Remove unused remnants from A2 cpu

Support for the A2 cpu was removed in commit fb5a5157 ("powerpc:
Remove platforms/wsp and associated pieces"), and the externs:
__setup_cpu_a2 and __restore_cpu_a2 are still around and unused, so
remove them.

Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>