gptimer_stop() function

- ESP_OK: Stop GPTimer successfully - ESP_ERR_INVALID_ARG: Stop GPTimer failed because of invalid argument - ESP_ERR_INVALID_STATE: Stop GPTimer failed because the timer is not in running. - ESP_FAIL: Stop GPTimer failed because of other error

This function will transit the timer state from "run" to "enable". This function is allowed to run within ISR context If `CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM` is enabled, this function will be placed in the IRAM by linker, makes it possible to execute even when the Flash Cache is disabled.