powerpc/64e: Tie PPC_BOOK3E_64 to PPC_FSL_BOOK3E

Since the IBM A2 CPU support was removed, see commit
fb5a5157 ("powerpc: Remove platforms/wsp and associated pieces"),
the only 64-bit Book3E CPUs we support are Freescale (NXP) ones.

However our Kconfig still allows configurating a kernel that has 64-bit
Book3E support, but no Freescale CPU support enabled. Such a kernel
would never boot, it doesn't know about any CPUs.

It also causes build errors, as reported by lkp, because
PPC_BARRIER_NOSPEC is not enabled in such a configuration:

  powerpc64-linux-ld: arch/powerpc/net/bpf_jit_comp64.o:(.toc+0x0):
  undefined reference to `powerpc_security_features'

To fix this, force PPC_FSL_BOOK3E to be selected whenever we are
building a 64-bit Book3E kernel.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220304061222.2478720-1-mpe@ellerman.id.au