xen/arm,arm64: rename __generic_dma_ops to xen_get_dma_ops

Now that __generic_dma_ops is a xen specific function, rename it to
xen_get_dma_ops. Change all the call sites appropriately.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: linux@armlinux.org.uk
CC: catalin.marinas@arm.com
CC: will.deacon@arm.com
CC: boris.ostrovsky@oracle.com
CC: jgross@suse.com
CC: Julien Grall <julien.grall@arm.com>