board: freescale: ls1021aqds: Update I2C mux config

Updates the board configuration to enable use of the PCA9547 I2C mux.

Signed-off-by: Stephen Carlson <stcarlso@linux.microsoft.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/configs/ls1021aqds_qspi_defconfig b/configs/ls1021aqds_qspi_defconfig
index 17a7a60..1f6f819 100644
--- a/configs/ls1021aqds_qspi_defconfig
+++ b/configs/ls1021aqds_qspi_defconfig
@@ -69,3 +69,4 @@
 CONFIG_USB_XHCI_HCD=y
 CONFIG_USB_XHCI_DWC3=y
 CONFIG_USB_STORAGE=y
+CONFIG_FSL_USE_PCA9547_MUX=y