renesas: rcar: Apply ATF overlay for reserved-memory

The function fdtdec_board_setup() is called early and adds the overlay
from ATF to the u-boot device tree. That is necessary so that u-boot
doesn't use reserved memory.

Linux also needs to know about that reserved memory so the overlay from
ATF needs to be aplied on the linux device tree as well.

This commit makes sure that the ATF overlay is applied to both device trees.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Adam Ford <aford173@gmail.com>