powerpc: Remove some of the legacy iSeries specific device drivers

These drivers are specific to the PowerPC legacy iSeries platform and
their Kconfig is specified in arch/powerpc.  Legacy iSeries is being
removed, so these drivers can no longer be selected.

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>