ARM: dts: imx53: Fix backlight for M53EVK

Remove the PWM backlight pin from the hog pins list, so it doesn't
collide with the PWM pin group. Moreover, add dummy regulator for
the backlight so that the backlight driver probes correctly.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>