arm: mach-k3: am642_init: Add missing ddr guard

The `struct udevice *` reference is needed for either of the
K3_LOAD_SYSFW, K3_AM64_DDRSS config guards. Adding the missing
K3_AM64_DDRSS guard.

Signed-off-by: Gowtham Tammana <g-tammana@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210624171614.14244-1-g-tammana@ti.com