pcnt_unit_start() function

- ESP_OK: Start PCNT unit successfully - ESP_ERR_INVALID_ARG: Start PCNT unit failed because of invalid argument - ESP_ERR_INVALID_STATE: Start PCNT unit failed because the unit is not enabled yet - ESP_FAIL: Start PCNT unit failed because of other error

This function should be called when the unit is in the enable state (i.e. after calling `pcnt_unit_enable()`) This function is allowed to run within ISR context This function will be placed into IRAM if `CONFIG_PCNT_CTRL_FUNC_IN_IRAM` is on, so that it's allowed to be executed when Cache is disabled