ARM: ux500: Add db8500 Device Tree node for misc/ab8500-pwm

This adds the DT node for the misc/ab8500-pwm driver. It will
allow probing of the driver during a Device Tree enabled boot.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>