ARM: dts: cubox-i: add support for PWM-driven front panel LED

The front panel LED on the Cubox-i is driven by one of the iMX6 PWM
channels, and is wired between the PWM output and supply.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>