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>