ARM: dts: Add pwm-fan node for exynos4412-odroidu3

Add pwm-fan node to the Odroid-U3 board file to enable PWM control of the
cooling fan. In addition, add the "pwm" label to the pwm@139D0000 node
in the exynos4412.dtsi.

Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>