gdma_set_priority() function

- ESP_OK: Set GDMA channel priority successfully - ESP_ERR_INVALID_ARG: Set GDMA channel priority failed because of invalid argument, e.g. priority out of range [0,GDMA_LL_CHANNEL_MAX_PRIORITY] - ESP_FAIL: Set GDMA channel priority failed because of other error

By default, all GDMA channels are with the same priority: 0. Channels with the same priority are served in round-robin manner.