ARM: dts: imx6sll: correct sdma compatible

Correct sdma compatible since ecspi errata ERR009165 has been fixed
on i.mx6sll as i.mx6ul.

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>