dmaengine: zynqmp_dma: add missing MODULE_LICENSE

We get a warning about the missing MODULE_LICENSE tag for this newly
added driver module:

WARNING: modpost: missing MODULE_LICENSE() in drivers/dma/xilinx/zynqmp_dma.o
see include/linux/module.h for more information

This adds a "GPL" license, matching the "version 2 or later" information in
the comment at the start of the file.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>