configs: am65x: Merge the HS and non-HS defconfigs

K3 devices have runtime type board detection. Make the default defconfig
include the secure configuration. Then remove the HS specific config.

Non-HS devices will continue to boot due to runtime device type detection.

Signed-off-by: Andrew Davis <afd@ti.com>
Tested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Manorit Chawdhry <m-chawdhry@ti.com>