Kconfig: Enable FIT_SIGNATURE if ARM64

Enabling FIT_SIGNATURE required the old authentication method to be
disabled so disable this for K3 SOCs and enable FIT_SIGNATURE for K3
Platforms.

Signed-off-by: Kamlesh Gurudasani <kamlesh@ti.com>
[ cleanup the patch ]
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 3045698..bdd535f 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -799,6 +799,7 @@
 	select SUPPORT_SPL
 	select FIT
 	select REGEX
+	select FIT_SIGNATURE if ARM64
 
 config ARCH_OMAP2PLUS
 	bool "TI OMAP2+"