powerpc: Remove orphaned reg_a2.h

Commit fb5a5157 ("powerpc: Remove platforms/wsp and associated
pieces") removed the A2 CPU support, but missed removal of reg_a2.h.

None of the defines contained in it are used, with the exception of the
SPRN_TEN* values, but they are also defined in reg_booke.h.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231113043947.1931831-1-mpe@ellerman.id.au