arm: Enable SYS_THUMB_BUILD on AT91

Several AT91 boards are quite close to their SPL size limit. For example,
sama5d27_wlsom1_ek_mmc is just 173 bytes short of its limit and doesn't
even fit with older GCCs.

All AT91 processors should have thumb support. Enable SYS_THUMB_BUILD. This
shrinks SPL by around 30%.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index a6f0018..1fd7aac 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -568,6 +568,7 @@
 	select GPIO_EXTRA_HEADER
 	select SPL_BOARD_INIT if SPL && !TARGET_SMARTWEB
 	select SPL_SEPARATE_BSS if SPL
+	imply SYS_THUMB_BUILD
 
 config ARCH_DAVINCI
 	bool "TI DaVinci"