arm: mach-k3: common: Default to non fitImage boot on HS-FS

Allow non fitImage bootflow on Field Securable (HS-FS) devices in
addition to GP, force fitImage boot only on Security enforced (HS-SE)
devices where signed images are necessary to maintain chain of trust.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Kamlesh Gurudasani <kamlesh@ti.com>