ARM: dts: imx6sll-evk: enable PWM1 for backlight driver

Enable pwm1 module on i.MX6SLL EVK board to make
backlight driver really work with LCD panel connected.

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>