powerpc/embedded6xx: select MPC10X_BRIDGE only if PCI is set

When CONFIG_SMP is not set, CONFIG_BROKEN_ON_SMP is set, and
CONFIG_PCI is not set, there can be a kconfig warning:

WARNING: unmet direct dependencies detected for PPC_INDIRECT_PCI
  Depends on [n]: PCI [=n]
  Selected by [y]:
  - MPC10X_BRIDGE [=y]

To fix that, make the selects of MPC10X_BRIDGE be conditional
on PCI and use "imply" instead of "select".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au> # use "imply"
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230521225103.19197-1-rdunlap@infradead.org