dmaengine: zxdma: Support ZTE ZX296702 dma

Add ZTE ZX296702 dma controller support. Only
device tree probe is support currently.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>