pwm: samsung: Update DT bindings documentation to cover clocks

PWM driver consumes at least one and up to three clocks, which need to be
specified in device tree when used. This patch updates bindings
documentation to add information about clocks.

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