gptimer_enable() function

- ESP_OK: Enable GPTimer successfully - ESP_ERR_INVALID_ARG: Enable GPTimer failed because of invalid argument - ESP_ERR_INVALID_STATE: Enable GPTimer failed because the timer is already enabled - ESP_FAIL: Enable GPTimer failed because of other error

This function will transit the timer state from "init" to "enable". This function will enable the interrupt service, if it's lazy installed in `gptimer_register_event_callbacks`. This function will acquire a PM lock, if a specific source clock (e.g. APB) is selected in the `gptimer_config_t`, while `CONFIG_PM_ENABLE` is enabled. Enable a timer doesn't mean to start it. See also `gptimer_start` for how to make the timer start counting.