arm: mach-k3: Enable dcache in SPL

Add support for enabling dcache already in SPL. It accelerates the boot
and resolves the risk to run into unaligned 64-bit accesses.

Based on original patch by Lokesh Vulta.

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