arm: u8500: Prefer building in thumb mode by default

Enabling CONFIG_SYS_THUMB_BUILD produces a significantly smaller
U-Boot binary (250 KiB vs 320 KiB) that still seems to be fully
functional. Make use of that by default but keep it as "imply" so it
can be disabled for testing in case this causes trouble for someone.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 18fcc7a..c815ad4 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1062,6 +1062,7 @@
 	imply PL01X_SERIAL
 	imply PMIC_AB8500
 	imply RTC_PL031
+	imply SYS_THUMB_BUILD
 	imply SYSRESET_SYSCON
 
 config ARCH_VERSAL