powerpc/32: Do not allow selection of e5500 or e6500 CPUs on PPC32

powerpc/32: Do not allow selection of e5500 or e6500 CPUs on PPC32

Commit 0e00a8c9fd92 ("powerpc: Allow CPU selection also on PPC32")
enlarged the CPU selection logic to PPC32 by removing depend to
PPC64, and failed to restrict that depend to E5500_CPU and E6500_CPU.
Fortunately that got unnoticed because -mcpu=8540 will override the
-mcpu=e500mc64 or -mpcu=e6500 as they are ealier, but that's
fragile and may no be right in the future.

Add back the depend PPC64 on E5500_CPU and E6500_CPU.

Fixes: 0e00a8c9fd92 ("powerpc: Allow CPU selection also on PPC32")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/8abab4888da69ff78b73a56f64d9678a7bf684e9.1657549153.git.christophe.leroy@csgroup.eu