dmaengine: zx: rename zx296702_dma.c to zx_dma.c

ZTE ZX dma driver is not ZX296702 specific.  It works for not only
ZX296702 but also other ZTE ZX family platforms like ZX296718.  Let's
rename the file to reflect that.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>