lib: Kconfig: k3: Enable SHA512 for fit signature

We are using our custMpk for signing that is a 4096 bit key, 4096 bit
rsa key requires a SHA512 hashing algorithm to be enabled as per the
source. Even though it is not mandated but this is how it works and is
tested.

Enables SHA512 if fit signature is enabled on K3 platforms.

Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>