powerpc/configs: Re-enable CONFIG_SCSI_DH

Commit 086b91d0 ("scsi_dh: integrate into the core SCSI code")
changed CONFIG_SCSI_DH from tristate to bool.

Our defconfigs have CONFIG_SCSI_DH=m, which the kconfig machinery warns
us is invalid, but instead of converting it to =y it leaves it unset.
This means we loose the CONFIG_SCSI_DH code and everything that depends
on it.

So convert the values in the defconfigs to =y.

Fixes: 086b91d0

 ("scsi_dh: integrate into the core SCSI code")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>