pwm: meson: add relative_duty interface in pwm sysfs [1/1]

pwm: meson: add relative_duty interface in pwm sysfs [1/1]

PD#SWPL-4225

Problem:
This interface is added to facilitate debugging the voltage-
table in pwm regulator, which can output different percentages
of waveforms in a fixed pwm cycle.
example:
echo 50 100 > sys/class/pwm/pwmchipx/pwmx/relative_duty
output 50% duty pwm

Solution:
add relative_duty interface in pwm sysfs

Verify:
test pass on tl1_X301_v1 tl1_skt_v1

Change-Id: Icd01e681694b81f3790f7cce8c326db02674a48c
Signed-off-by: Bichao Zheng <bichao.zheng@amlogic.com>