esp_adc: add adc ulp mode disable enum

Prior to this commit, adc ulp is disabled by setting
adc_oneshot_unit_init_cfg_t::ulp_mode to false.

After this commit, a new enum `ADC_ULP_MODE_DISABLE` is added. So
setting `ulp_mode` to `ADC_ULP_MODE_DISABLE`, instead of `false`, to
disable the ulp mode.

examples/peripherals/adc/oneshot_read/main/oneshot_read_main.c

@@ -70,7 +70,7 @@ void app_main(void)
     adc_oneshot_unit_handle_t adc2_handle;
     adc_oneshot_unit_init_cfg_t init_config2 = {
         .unit_id = ADC_UNIT_2,
-        .ulp_mode = false,
+        .ulp_mode = ADC_ULP_MODE_DISABLE,
     };
     ESP_ERROR_CHECK(adc_oneshot_new_unit(&init_config2, &adc2_handle));