mx6qsabresd: Document the mapping of USDHC[2-4]

This documents the SD card identifier so it is easier for user to spot
which card number will be used, if need.
Signed-off-by: NOtavio Salvador <otavio@ossystems.com.br>

board/freescale/mx6qsabresd/mx6qsabresd.c

@@ -160,6 +160,13 @@ int board_mmc_init(bd_t *bis)
 {
 	int i;
 
+	/*
+	 * According to the board_mmc_init() the following map is done:
+	 * (U-boot device node)    (Physical Port)
+	 * mmc0                    SD2
+	 * mmc1                    SD3
+	 * mmc2                    eMMC
+	 */
 	for (i = 0; i < CONFIG_SYS_FSL_USDHC_NUM; i++) {
 		switch (i) {
 		case 0: