ARM: dts: exynos4: Specify PWM clocks in PWM node

Since pwm-samsung bindings require at least one clock to be specified,
this patch adds the missing clocks and clock-names properties to specify
clocks used by PWM block on Exynos4 SoCs.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>