nvmem: imx-ocotp: add support for imx6sll

i.MX6SLL is a new SoC of i.MX6 family, enable ocotp
driver support for this SoC.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>