clk: pwm: implement the .get_duty_cycle callback

Commit 9fba738a ("clk: add duty cycle support") added support for
getting and setting the duty cycle of a clock. This implements the
get_duty_cycle callback for PWM based clocks so the duty cycle is shown
in the debugfs output (/sys/kernel/debug/clk/clk_summary).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>