1.Putting the PWM_CHANNEL_X, PWM_PRESCALER_CX information
in the description.
2.Use the common definition EC_FREQ.
3.Use the common macro IT8XXX2_DT_ALT_ITEMS_LIST.
4.Stop using DRV_CONFIG, DRV_REG macro.
Signed-off-by: Ruibin Chang <Ruibin.Chang@ite.com.tw>