boards: arm: Add PWM LEDs to the mimxrt685_evk board

Adds support for PWM LEDs (red, green, blue) to the mimxrt685_evk board
by adding devicetree nodes and aliases, and configuring the associated
pinmuxes. The red PWM LED is disabled by default because it's connected
to the same PWM channel as the blue PWM LED.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>