arm: k3: Consolidate and silence k3_fit_atf.sh call

Buiding u-boot-spl-k3[_HS].its is currently unconditionally verbose
about what it does. Change that by wrapping the call to k3_fit_atf.sh
into a cmd, also using that chance to reduce duplicate lines of makefile
code - only IS_HS=1 is different when CONFIG_TI_SECURE_DEVICE is on.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Lokesh Vutla <lokeshvutla@ti.com>