[ARM] Refine selection of ISA_DMA_API and generic dma.c code

ISA_DMA_API tells the rest of the kernel if the ISA DMA API is
available.  Select this symbol only on machine types which make
use of the ISA DMA API.

Make building of arch/arm/kernel/dma.c depend on this symbol -
if a machine does not support the ISA DMA API, it's pointless
building this file.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>