powerpc: Fix GENERIC_ISA_DMA dependency

On PowerPC we should always use generic ISA DMA API implementation
as there is simply no other implementation exist.

Without this patch, the following build error pops up:

  sound/built-in.o: In function 'snd_dma_pointer':
  (.text+0x74ae): undefined reference to 'dma_spin_lock'
  ...
  make: *** [.tmp_vmlinux1] Error 1

This is PPC_85xx, SMP and some sound drivers set to =y.

Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>
Acked-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>