clocksource: samsung_pwm_timer: Get clock from device tree

When booting with device tree static clkdev aliases should not be used.
This patch modifies the samsung_pwm_timer driver to use DT-based clock
lookup when booting with device tree.

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>