ARM: imx: remove i.MX6SLL support in i.MX6SL cpu idle driver

i.MX6SLL supports ARM power off in cpu idle, better to reuse
i.MX6SX cpu idle driver instead of i.MX6SL which does NOT
support ARM power off.

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>