ARM: dts: fix pwm-cells in pwm node for exynos4

pwm-cells should be 3. Third cell is optional PWM flags. And This flag
supported by this binding is PWM_POLARITY_INVERTED.

Signed-off-by: Jaewon Kim <jaewon02.kim@samsung.com>
Reviewed-by: Sachin Kamat <sachin.kamat@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>