dmaengine: pl08x: Make slave engine optional

If the vendor data does not specify any signals, we do not
have to support slave DMA. Make the registration of the slave
DMA engine optional, so we can use this for the FTDMAC020
in the Gemini that only has memcpy support.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>