arm: Select CONFIG_ARM64 for Cavium ThunderX

Select the config option, since this board is ARM64.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 7a77b6a..e5463d2 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -766,6 +766,7 @@
 
 config TARGET_THUNDERX_88XX
 	bool "Support ThunderX 88xx"
+	select ARM64
 	select OF_CONTROL
 
 endchoice