mx6sxsabresd: Convert to the new Kconfig style

mx6sxsabresd was not in the master branch when the conversion to the new Kconfig
style happened, so convert it now so that it can build again.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 63f8576..f01ff8f 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -527,6 +527,9 @@
 config TARGET_MX6SLEVK
 	bool "Support mx6slevk"
 
+config TARGET_MX6SXSABRESD
+	bool "Support mx6sxsabresd"
+
 config TARGET_GW_VENTANA
 	bool "Support gw_ventana"
 
@@ -884,6 +887,7 @@
 source "board/freescale/mx6qsabreauto/Kconfig"
 source "board/freescale/mx6sabresd/Kconfig"
 source "board/freescale/mx6slevk/Kconfig"
+source "board/freescale/mx6sxsabresd/Kconfig"
 source "board/freescale/vf610twr/Kconfig"
 source "board/gateworks/gw_ventana/Kconfig"
 source "board/genesi/mx51_efikamx/Kconfig"