ARM: i.MX6: TARGET_NITROGEN6X: add 'select MX6QDL'

This fixes commit <91435cd4> "ARM: i.MX6: exclude the ARM errata
 from i.MX6 UP system"

for nitrogen6x. The above commit removed the errata for the board
since MX6Q/MXDL/MX6S is selected via CONFIG_SYS_EXTRA_OPTIONS

This restores the errata configs.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>

arch/arm/mach-imx/mx6/Kconfig

@@ -424,6 +424,7 @@ config TARGET_NITROGEN6X
 	imply USB_ETHER_MCS7830
 	imply USB_ETHER_SMSC95XX
 	imply USB_HOST_ETHER
+	select MX6QDL
 
 config TARGET_OPOS6ULDEV
 	bool "Armadeus OPOS6ULDev board"