ARM: dts: imx6sll: declare src module to be compatible to imx51's src

i.MX6SLL uses same SRC module as i.MX51, add "fsl,imx51-src"
compatible string to enable SRC driver to support setting
CPU resume address for cpu-idle and suspend/resume.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>