configs: am64x_evm_a53_defconfig: Switch to per-cpu timer as tick provider

On arm64 systems, recommendation is to use per-cpu timer for time
keeping. Currently AM64 ends up using DM timer as tick timer as driver
is enabled in the config. Drop OMAP DM Timer related configs, this will
switch to using armv8 per-cpu timer as tick timer for A53 SPL/U-Boot.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-By: Nishanth Menon <nm@ti.com>