powerpc/86xx: Drop unused CONFIG_MPC8610

The MPC8610 symbol used to be default y if MPC8610_HPCD, but since
MPC8610_HPCD was removed MPC8610 is now never used. Remove it.

Fixes: 248667f8 ("powerpc: drop HPCD/MPC8610 evaluation platform support")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231123032902.2760818-1-mpe@ellerman.id.au