gptimer_start() function

- ESP_OK: Start GPTimer successfully - ESP_ERR_INVALID_ARG: Start GPTimer failed because of invalid argument - ESP_ERR_INVALID_STATE: Start GPTimer failed because the timer is not enabled or is already in running - ESP_FAIL: Start GPTimer failed because of other error

This function will transit the timer state from "enable" to "run". 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.