board: ti: am654: select SYS_DISABLE_DCACHE_OPS for arm64 build

AM654 SoC is IO coherent wrt A53 cores, therefore enable
SYS_DISABLE_DCACHE_OPS to avoid cache operations in A53
SPL/U-Boot.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>