configs: Make TI_SECURE_DEVICE default for K3

All K3 boards now are secure by default, instead of setting this in each
defconfig, make it implied by the ARCH config.

The only exception is IOT2050, which I do not believe will have any
problems with being a TI_SECURE_DEVICE, but for now turn it off to keep
its config the same.

Signed-off-by: Andrew Davis <afd@ti.com>
Tested-by: Tom Rini <trini@konsulko.com>