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>

arch/arm/Kconfig

@@ -766,6 +766,7 @@ config ARCH_ROCKCHIP
 
 config TARGET_THUNDERX_88XX
 	bool "Support ThunderX 88xx"
+	select ARM64
 	select OF_CONTROL
 
 endchoice