gdma_new_axi_channel() function

- ESP_OK: Create DMA channel successfully - ESP_ERR_INVALID_ARG: Create DMA channel failed because of invalid argument - ESP_ERR_NO_MEM: Create DMA channel failed because out of memory - ESP_FAIL: Create DMA channel failed because of other error

This API won't install interrupt service for the allocated channel. If interrupt service is needed, user has to register GDMA event callback by `gdma_register_tx_event_callbacks` or `gdma_register_rx_event_callbacks`.