ARM: SAMSUNG: Add clk enable/disable of pwm

PWM timers use pclk("timers" clk) as parent clk. If this pclk is the
disabled state when PWM driver is probed, then it causes wrong read and
write operation about registers of PWM.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>