ARM: dts: cygnus: Add the PWM node

This is connected up to the backlight on 911360_entphn, which we'll
need for a panel driver. For now, leave the node disabled in the
shared dtsi.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>