powerpc/pseries: Remove redundant select of PPC_DOORBELL

Commit d4e58e59

 ("powerpc/powernv: Enable POWER8 doorbell IPIs")
added a select of PPC_DOORBELL to PPC_PSERIES, but it already had a
select of PPC_DOORBELL. One is enough.

Reported-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191219125840.32592-1-mpe@ellerman.id.au