powerpc/64s: Add FA_DUMP to defconfig

FA_DUMP (Firmware Assisted Dump) is a powerpc only feature that should
be enabled in our defconfig to get some build / test coverage.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210420042209.1641634-1-mpe@ellerman.id.au