ARM: mx6: Enable Thumb build for SPL

Building the SPL in Thumb mode saves roughly 30% in size of the
resulting SPL binary. As the size of SPL it limited on the MX6,
this helps a lot.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Tim Harvey <tharvey@gateworks.com>

include/configs/imx6_spl.h

@@ -24,6 +24,7 @@
  *    and some padding thus 'our' max size is really 0x00908000 - 0x00918000
  *    or 64KB
  */
+#define CONFIG_SYS_THUMB_BUILD
 #define CONFIG_SPL_LDSCRIPT	"arch/arm/cpu/armv7/omap-common/u-boot-spl.lds"
 #define CONFIG_SPL_TEXT_BASE		0x00908000
 #define CONFIG_SPL_MAX_SIZE		(64 * 1024)