boards: arm64: enable ARMV8_A_NS by default for R-Car boards

We need to enable this configuration for all R-Car ARM64 boards.
First and foremost, we definitely should run Zephyr on the boards
in the NS-EL1 state. The EL3 is used for TF-A, EL2 is used for
U-Boot, and Xen in the case when we run Zephyr as Dom-0. The S-EL1
is used for OP-Tee, and S-EL0 is used for OP-Tee apps.

Signed-off-by: Mykola Kvach <mykola_kvach@epam.com>