pwm: pwm-tiehrpwm: Update the clock handling of pwm-tiehrpwm driver

The clock framework has changed and it's now better to invoke
clock_prepare_enable() and clk_disable_unprepare() rather than the
legacy clk_enable() and clk_disable() calls. This patch converts the
pwm-tiehrpwm driver to the new framework.

Signed-off-by: Philip Avinash <avinashphilip@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>