kconfig: imx6: add SUPPORT_SPL

add SUPPORT_SPL feature for iMX6 SabreSD. It need to use
mx6sabresd_spl_defconfig to compile it.

Signed-off-by: John Tobias <john.tobias.ph@gmail.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 79ccc06..b00af7b 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -609,6 +609,7 @@
 config TARGET_MX6SABRESD
 	bool "Support mx6sabresd"
 	select CPU_V7
+	select SUPPORT_SPL
 
 config TARGET_MX6SLEVK
 	bool "Support mx6slevk"